tidy-up: misc build, tests, `lib/macos.c`

- mkhelp.pl: drop unused assigment.
- Makefile.mk: update a comment.
- lib/macos.c: delete redundant block.
- fix two typos.

Cl

tidy-up: misc build, tests, `lib/macos.c`

- mkhelp.pl: drop unused assigment.
- Makefile.mk: update a comment.
- lib/macos.c: delete redundant block.
- fix two typos.

Closes #14558

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macos: undo `availability` macro enabled by Homebrew gcc

Homebrew gcc builds starting with 12.4.0, 13.3.0 and 14.1.0 enabled
the `availability` attribute.

This broke builds beca

macos: undo `availability` macro enabled by Homebrew gcc

Homebrew gcc builds starting with 12.4.0, 13.3.0 and 14.1.0 enabled
the `availability` attribute.

This broke builds because the way the Apple SDK uses attributes (when
available) are incompatible with how gcc accepts them. Causing these
errors:
```
error: attributes should be specified before the declarator in a function definition
error: expected ',' or '}' before
```

Upstream commits implementing the `availability` macro:
gcc-12: https://github.com/iains/gcc-12-branch/commit/fd5530b7cb0012bf4faeddd45e13054a1dfa6783
gcc-13: https://github.com/iains/gcc-13-branch/commit/cb7e4eca68cfc4763474e2eb0935a844458842a8
gcc-14: https://github.com/iains/gcc-14-branch/commit/ff62a108865a6403f5017380d7018250c1d3306f

The project above is a Darwin gcc compatibility pack, that is applied
to Homebrew gcc builds.

This patch works by redefining the `availability` macro to an invalid
value, making `__has_attribute(availability)` checks fail, stopping
Apple SDK from inserting the incompatible attributes.

It also replaces the previous, local workaround for `lib/macos.c`.

Example with gcc 12.4.0 with macOS SDK 14.0 (Xcode 15.0.1):
```
In file included from <path-to-sdk>/MacOSX14.0.sdk/System/Library/Frameworks/CoreFoundation.framework/Headers/CoreFoundation.h:54,
from <path-to-sdk>/MacOSX14.0.sdk/System/Library/Frameworks/SystemConfiguration.framework/Headers/SCDynamicStoreCopySpecific.h:30,
from /Users/runner/work/curl/curl/lib/macos.c:33,
from /Users/runner/work/curl/curl/build/lib/CMakeFiles/libcurl_shared.dir/Unity/unity_0_c.c:244:
<path-to-sdk>/MacOSX14.0.sdk/System/Library/Frameworks/CoreFoundation.framework/Headers/CFUserNotification.h:126:1: error: attributes should be specified before the declarator in a function definition
126 | CF_INLINE CFOptionFlags CFUserNotificationCheckBoxChecked(CFIndex i) API_AVAILABLE(macos(10.0)) API_UNAVAILABLE(ios, watchos, tvos) {return ((CFOptionFlags)(1UL << (8 + i)));}
| ^~~~~~~~~
```
Ref: https://github.com/curl/curl/actions/runs/9787982387/job/27025351601?pr=14096#step:7:18

The gcc vs. llvm/clang incompatibility possibly tracked here upstream:
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108796
More info:
https://github.com/llvm/llvm-project/issues/81767
https://github.com/gcc-mirror/gcc/commit/8433baadec88e5f31fa141b6d78094e91256079d
https://discourse.llvm.org/t/changing-attribute-ast-printing-location-for-gcc-compatibility/73215
https://reviews.llvm.org/D159362

Follow-up to db135f8d7207b20d531e7e2100a49f3e16bdcfab #14119
Ref: https://github.com/curl/curl/pull/14091#issuecomment-2222703468
Fixes #13700
Cherry-picked from #14097
Closes #14155

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macos: add workaround for gcc, non-c-ares, IPv6, compile error

Apple macOS SDK 13.0 and later are increasingly incompatible with gcc,
which started causing CI errors with the 20240701.9

macos: add workaround for gcc, non-c-ares, IPv6, compile error

Apple macOS SDK 13.0 and later are increasingly incompatible with gcc,
which started causing CI errors with the 20240701.9 revision of the
`macos-latest` (= `macos-14-arm64`) runner image.

This error is happening inside an Apple SDK header. We use the header
for calling a function in a resolver-related hack, in non-c-ares, IPv6
builds. You can avoid the problem by using c-ares or disabling IPv6
(or using clang, llvm, or a compatible gcc + SDK combination).

This patch fixes affected builds by declaring the ncessary framework
function manually, and not including the problematic header.

This workaround is ugly, doesn't cover all combinations, and fragile.

Other options are to disable this resolver-related hack for GCC, or to
replace it with a solution that doesn't rely on Apple SDK.

If you are aware of a stable fix or workaround, let us know.

gcc 12.4.0 + macOS SDK 14.0 (Xcode 15.0.1) error example:
```
In file included from /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX14.0.sdk/System/Library/Frameworks/CoreFoundation.framework/Headers/CoreFoundation.h:54,
from /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX14.0.sdk/System/Library/Frameworks/SystemConfiguration.framework/Headers/SCDynamicStoreCopySpecific.h:30,
from /Users/runner/work/curl/curl/lib/macos.c:33,
from /Users/runner/work/curl/curl/build/lib/CMakeFiles/libcurl_shared.dir/Unity/unity_0_c.c:244:
/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX14.0.sdk/System/Library/Frameworks/CoreFoundation.framework/Headers/CFUserNotification.h:126:1: error: attributes should be specified before the declarator in a function definition
126 | CF_INLINE CFOptionFlags CFUserNotificationCheckBoxChecked(CFIndex i) API_AVAILABLE(macos(10.0)) API_UNAVAILABLE(ios, watchos, tvos) {return ((CFOptionFlags)(1UL << (8 + i)));}
| ^~~~~~~~~
/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX14.0.sdk/System/Library/Frameworks/CoreFoundation.framework/Headers/CFUserNotification.h:127:1: error: attributes should be specified before the declarator in a function definition
127 | CF_INLINE CFOptionFlags CFUserNotificationSecureTextField(CFIndex i) API_AVAILABLE(macos(10.0)) API_UNAVAILABLE(ios, watchos, tvos) {return ((CFOptionFlags)(1UL << (16 + i)));}
| ^~~~~~~~~
/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX14.0.sdk/System/Library/Frameworks/CoreFoundation.framework/Headers/CFUserNotification.h:128:1: error: attributes should be specified before the declarator in a function definition
128 | CF_INLINE CFOptionFlags CFUserNotificationPopUpSelection(CFIndex n) API_AVAILABLE(macos(10.0)) API_UNAVAILABLE(ios, watchos, tvos) {return ((CFOptionFlags)(n << 24));}
| ^~~~~~~~~
```
Ref: https://github.com/curl/curl/actions/runs/9787982387/job/27025351601?pr=14096#step:7:18

The exact conditions are fuzzy. Oddly enough gcc 12.3.0 and the SDK
same as above are _compatible_:
https://github.com/curl/curl/actions/runs/9791701214/job/27036037162

Also notice that similar errors can also happen in SecureTransport
builds, due to the SDK headers required.

Ref: https://github.com/curl/curl/pull/14097#issuecomment-2208639046
Ref: https://github.com/curl/curl/pull/14091#issuecomment-2205870854
Cherry-picked from #14097
Closes #14119

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code: language cleanup in comments

Based on the standards and guidelines we use for our documentation.

- expand contractions (they're => they are etc)
- host name = > hostname

code: language cleanup in comments

Based on the standards and guidelines we use for our documentation.

- expand contractions (they're => they are etc)
- host name = > hostname
- file name => filename
- user name = username
- man page => manpage
- run-time => runtime
- set-up => setup
- back-end => backend
- a HTTP => an HTTP
- Two spaces after a period => one space after period

Closes #14073

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macOS: fix target detection more

Now SCDynamicStoreCopyProxies is called (and the required frameworks are
linked in) on all versions of macOS and only on macOS. Fixes crash due
to un

macOS: fix target detection more

Now SCDynamicStoreCopyProxies is called (and the required frameworks are
linked in) on all versions of macOS and only on macOS. Fixes crash due
to undefined symbol when built with the macOS 10.11 SDK or earlier.

CURL_OSX_CALL_COPYPROXIES is renamed to CURL_MACOS_CALL_COPYPROXIES and
is now only defined when SCDynamicStoreCopyProxies will actually be
called. Previously, it was defined when ENABLE_IPV6 was not defined but
SCDynamicStoreCopyProxies is not called in that case.

TARGET_OS_OSX is only defined in the macOS 10.12 SDK and later and only
when dynamic targets are enabled. TARGET_OS_MAC is always defined but
means any Mac OS or derivative including macOS, iOS, tvOS, and watchOS.
TARGET_OS_IPHONE means any Darwin OS other than macOS.

Follow-up to c73b2f82

Fixes #11502
Closes #11516

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macOS: fix taget detection

- TARGET_OS_OSX is not always defined on macOS
- this leads to missing symbol Curl_macos_init()
- TargetConditionals.h seems to define these only when

macOS: fix taget detection

- TARGET_OS_OSX is not always defined on macOS
- this leads to missing symbol Curl_macos_init()
- TargetConditionals.h seems to define these only when
dynamic targets are enabled (somewhere?)
- this PR fixes that on my macOS 13.4.1
- I have no clue why CI builds worked without it

Follow-up to c7308592fb8ba213fc2c1
Closes #11417

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hostip.c: Move macOS-specific calls into global init call

https://github.com/curl/curl/pull/7121 introduced a macOS system call
to `SCDynamicStoreCopyProxies`, which is invoked every tim

hostip.c: Move macOS-specific calls into global init call

https://github.com/curl/curl/pull/7121 introduced a macOS system call
to `SCDynamicStoreCopyProxies`, which is invoked every time an IP
address needs to be resolved.

However, this system call is not thread-safe, and macOS will kill the
process if the system call is run first in a fork. To make it possible
for the parent process to call this once and prevent the crash, only
invoke this system call in the global initialization routine.

In addition, this change is beneficial because it:

1. Avoids extra macOS system calls for every IP lookup.
2. Consolidates macOS-specific initialization in a separate file.

Fixes #11252
Closes #11254

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