cmake: clear package version after `pkg-config` detection

`pkg_check_modules()` seems to leave `<PACKAGE>_VERSION` defined with an
empty value, if the package is not found.

When

cmake: clear package version after `pkg-config` detection

`pkg_check_modules()` seems to leave `<PACKAGE>_VERSION` defined with an
empty value, if the package is not found.

When the package is also not found in the fallback branch,
`find_package_handle_standard_args()` logs and error message. In this
message it includes the bogus empty value as: `(found version "")`:
```
Could NOT find Libssh2 (missing: LIBSSH2_INCLUDE_DIR LIBSSH2_LIBRARY) (found version "")
```
https://github.com/curl/curl/actions/runs/11509727553/job/32040378958?pr=15408#step:31:99

Clear the version number to avoid the confusion:
```
Could NOT find Libssh2 (missing: LIBSSH2_INCLUDE_DIR LIBSSH2_LIBRARY)
```
https://github.com/curl/curl/actions/runs/11510022503/job/32041149129?pr=15408#step:31:99

Seen with CMake v3.30.5.

Follow-up to 7bab201abe3915a0167c002f9308950cb8a06e4b #15193
Closes #15409

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cmake: document `-D` and env build options

Extend `INSTALL-CMAKE` document with the list of available options,
a short description and default values.

The list may not be 100% c

cmake: document `-D` and env build options

Extend `INSTALL-CMAKE` document with the list of available options,
a short description and default values.

The list may not be 100% complete.

There are no component boundaries in CMake, so the line is blurry
between curl options, CMake options, CMake Find modules options.
I included certain CMake options that seemed useful, and/or have
dedicated use withing curl's CMake source. But, all CMake built-in
options are usable, as documented upstream in CMake.

The naming of the options has a heritage and the inconsistencies with
it, including a lack of clear namespace. This may be subject to future
updates, also after figuring out which name has special meaning within
CMake and/or CMake projects out of unwritten convention or something
more tangible.

CMake allows to initialize any internal variable via `-D`. This may be
useful to pre-initialize/override feature check results. The list
doesn't contain these, and they remain officially undocumented.

Also:
- make adjustments to keep the spellchecker happy.
- retrofit description changes to the cmake sources.
- stop documenting deprecated `Find*` variables.

Reported-by: Daniel Stenberg
Fixes https://github.com/curl/curl/discussions/14885
Closes #15388

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cmake: respect cflags/libdirs of native pkg-config detections

In Find modules with native pkg-config detection (libgsasl, libidn2,
libssh, libuv, nettle) use the C compiler flags returne

cmake: respect cflags/libdirs of native pkg-config detections

In Find modules with native pkg-config detection (libgsasl, libidn2,
libssh, libuv, nettle) use the C compiler flags returned by pkg-config.
Also use the library paths, and return the pathless library names.

Also:
- add these library paths to `libcurl.pc`/`curl-config`.
- fix libgsasl detection to use the detected header directory.

FindGSS already did this before this patch.

Fixes #14641

Closes #14652

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cmake: pkg-config 'found' message sync with native CMake

Cherry-picked from #14610

cmake: fix Find module and package names

- fix BearSSL warning about name mismatch.
- fix Nettle Find module not found on Linux.
- tidy-up: drop quotes from a package name.

cmake: fix Find module and package names

- fix BearSSL warning about name mismatch.
- fix Nettle Find module not found on Linux.
- tidy-up: drop quotes from a package name.

Package names must match case-sensitively to work on all platforms:
- `find_package(<NAME> ...)` in `CMakeLists.txt`.
- `CMake/Find<NAME>.cmake` filenames.
- `find_package_handle_standard_args(<NAME> ...` in Find modules.
- `message(STATUS "Found <NAME> ...` in Find modules.
(to match the message shown by `find_package_handle_standard_args()`)

Closes #14599

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cmake/FindNettle: log message when found via `pkg-config`

The message mimics the CMake-native message (by
`find_package_handle_standard_args()`), with the header path and version number.

cmake/FindNettle: log message when found via `pkg-config`

The message mimics the CMake-native message (by
`find_package_handle_standard_args()`), with the header path and version number.

Closes #14596

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cmake/FindNettle: skip `pkg-config` for custom configs

If either `NETTLE_INCLUDE_DIR` or `NETTLE_LIBRARY` is set to customize
the `nettle` dependency, skip `pkg-config` and use the CMake

cmake/FindNettle: skip `pkg-config` for custom configs

If either `NETTLE_INCLUDE_DIR` or `NETTLE_LIBRARY` is set to customize
the `nettle` dependency, skip `pkg-config` and use the CMake-native
detection to honor these custom settings.

Closes #14584

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cmake: tidy up more in Find modules

- add `NAMES` where missing.
- document input variables (including deprecated ones.)
- comment cleanups.
- FindWolfSSL: drop stray `QUIET` fro

cmake: tidy up more in Find modules

- add `NAMES` where missing.
- document input variables (including deprecated ones.)
- comment cleanups.
- FindWolfSSL: drop stray `QUIET` from `pkg_check_modules()`.
(`QUIET` may be re-added for all modules in the future.)

Closes #14579

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cmake: sync up version detection in Find modules

- use the same pattern across all Find modules:
- verify if the version header exists before reading it.
- use a single regex per

cmake: sync up version detection in Find modules

- use the same pattern across all Find modules:
- verify if the version header exists before reading it.
- use a single regex per lookup.
- sync regexes between Find modules.
- use generic temporary variable names.
- improve readability.
- make it simpler to transition to new CMake syntax in the future:
```cmake
file(STRINGS "${CARES_INCLUDE_DIR}/ares_version.h" _version_str REGEX "<...>")
unset(_version_str)
set(CARES_VERSION "${CMAKE_MATCH_1}")
```
Ref: https://cmake.org/cmake/help/latest/policy/CMP0159.html#policy:CMP0159

- fix zstd version detection to be CMake 3.7 compatible.
Required 3.9 before this patch, for the `CMAKE_MATCH_<n>` feature.
Follow-up to c5d506e9bbf0669d7605d1d00865ae7e229b2409 #12200

Follow-up to 4e2f3641f80e233738147d82218317f331369c50 #14548

Closes #14572

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cmake: revert to `pkg_check_modules()`

Prefer `pkg_check_modules()` over `pkg_search_module()`.

`pkg_check_modules()` logs a line when there is a hit, and also warnings
if a sub

cmake: revert to `pkg_check_modules()`

Prefer `pkg_check_modules()` over `pkg_search_module()`.

`pkg_check_modules()` logs a line when there is a hit, and also warnings
if a sub-dependency is missing. In `QUIET` mode, both are silent.

The extra info is useful to see if a detection happened via
`pkg-config`.

Keep `pkg_search_module()` in `FindGSS`. We pass two dependencies
there and we want to keep stopping on the first one.

Partially reverts c2889a7b4180fc963ae30811f59ab547b6eb03cd #14388

Closes #14573

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cmake: sync up formatting in Find modules

- lowercase internal variable names (FindGSS)
- comments
- whitespace

Closes #14527

cmake: add `CURL_USE_PKGCONFIG` option

Add option to control whether to use `pkg-config` to detect
dependencies. Curl's CMake uses `pkg-config` by default for all targets
except for

cmake: add `CURL_USE_PKGCONFIG` option

Add option to control whether to use `pkg-config` to detect
dependencies. Curl's CMake uses `pkg-config` by default for all targets
except for MSVC without vcpkg.

With the CMake option `-DCURL_USE_PKGCONFIG=ON` you can override it to
use `pkg-config` always.

If `pkg-config` is causing issues, e.g. in cross-builds or other cases,
`-DCURL_USE_PKGCONFIG=OFF` disables all use of `pkg-config`.

Also add it to `curl-config.cmake`. Not yet used, but will be once curl
starts referencing any curl-specific `Find*` module from this public
script.

Follow-up to 9dfdc6ff42ba045ec48056bb6d2072f2fcac2e9d #14483
Closes #14504

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cmake: allow `pkg-config` in more envs

Before this patch, `pkg-config` was used for `UNIX` builds only (with
a few exceptions like wolfSSL, libssh, gsasl, libuv). This patch extends

cmake: allow `pkg-config` in more envs

Before this patch, `pkg-config` was used for `UNIX` builds only (with
a few exceptions like wolfSSL, libssh, gsasl, libuv). This patch extends
`pkg-config` use to all envs except: `MSVC` without vcpkg. Meaning MSVC
with vcpkg will now use it. Also mingw on Windows.

Also apply the new condition to options where `pkg-config` was used
unconditionally (= for all targets). These are:
`-DCURL_USE_WOLFSSL=ON`, `-DCURL_USE_LIBSSH=ON`,
`-DCURL_USE_GSASL=ON` and `-DCURL_USE_LIBUV=ON`

This patch may still cause regressions for cross-builds (e.g. mingw
cross-build from Unix) and potentially other cases. If that happens, we
recommend using some of these methods to explicitly disable `pkg-config`
when using CMake:
- CMake option: `-DPKG_CONFIG_EXECUTABLE=`
(or `-DPKG_CONFIG_EXECUTABLE=nonexistent` or similar)
This is similar to the (curl-specific) `PKG_CONFIG` env for autotools.
- export env: `PKG_CONFIG_LIBDIR=`
(or `PKG_CONFIG_PATH`, `PKG_CONFIG_SYSROOT_DIR`,
or the CMake-specific `PKG_CONFIG`)

We may improve control over this in a future patch, also allowing opting
in MSVC (without vcpkg).

Ref: #14405
Ref: #14408
Ref: #14140
Closes #14483

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cmake: more syntax tidy-up

- quote string literals.
In the hope it improves syntax-highlighting and readability.

- use lowercase, underscore-prefixed local var names.
As

cmake: more syntax tidy-up

- quote string literals.
In the hope it improves syntax-highlighting and readability.

- use lowercase, underscore-prefixed local var names.
As a hint for scope, to help readability.

- prefer `pkg_search_module` (over `pkg_check_modules`).
They are the same, but `pkg_search_module` stops searching
at the first hit.

- more `IN LISTS` in `foreach()`.

- OtherTests.cmake: clear `CMAKE_EXTRA_INCLUDE_FILES` after use.

- add `PROJECT_LABEL` for http/client and unit test targets.

- sync `Find*` module comments and formatting.

- drop a few local variables.

- drop bogus `CARES_LIBRARIES` from comment.

- unquote numeric literal.

Follow-up to acbc6b703f6b0ee568d053f6f2565fbc107b5fd3 #14197
Closes #14388

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cmake: tidy-ups

- tidy-up comments.
- use lowercase, underscore prefixed names for internal variables.
- use `IN LISTS` and `IN ITEMS` in `foreach()` loops.
- rename variable nam

cmake: tidy-ups

- tidy-up comments.
- use lowercase, underscore prefixed names for internal variables.
- use `IN LISTS` and `IN ITEMS` in `foreach()` loops.
- rename variable name `OUTPUT` to a more distinctive one.
- tidy-up `STREQUAL` syntax.
- delete commented code.
- indent/whitespace.

Closes #14197

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cmake: drop `if(PKG_CONFIG_FOUND)` guard for `pkg_check_modules()`

The oldest cmake supported by curl is v3.7.0, which already has such
guard (using `PKG_CONFIG_EXECUTABLE`) inside `pkg_

cmake: drop `if(PKG_CONFIG_FOUND)` guard for `pkg_check_modules()`

The oldest cmake supported by curl is v3.7.0, which already has such
guard (using `PKG_CONFIG_EXECUTABLE`) inside `pkg_check_modules()`. The
advantage of leaving that guard to CMake is that it will define/reset
all output variables, while the manual guard doesn't do this and also
leaves for example `NETTLE_FOUND` undefined.

Delete the single use of this guard from the recently added `nettle`
detection, where I included it by accident. Then possibly re-introduce
it universally if we find it useful after more evaluation.

Follow-up to 669ce42275635dc1f881dab3dfc9a55c9ab49b21 #14285
Closes #14309

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cmake: detect `nettle` when building with GnuTLS

`nettle` is a direct dependency of curl, when building with GnuTLS.
Add a new `Find` module to detect it.

Also:
- GHA/macos:

cmake: detect `nettle` when building with GnuTLS

`nettle` is a direct dependency of curl, when building with GnuTLS.
Add a new `Find` module to detect it.

Also:
- GHA/macos: drop `nettle` hack no longer necessary.
- add `nettle` to `libcurl.pc`.
- also add `nettle` to `libcurl.pc` in autotools builds.

Follow-up to 781242ffa44a9f9b95b6da5ac5a1bf6372ec6257 #11967
Closes #14285

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