demos: tidy up makefiles, fix warnings

Update makefiles so that consistent patterns are used. Object files
are compiled from source using an implicit rule (but using our
CFLAGS); fo

demos: tidy up makefiles, fix warnings

Update makefiles so that consistent patterns are used. Object files
are compiled from source using an implicit rule (but using our
CFLAGS); for linking, we give an explicit rule. Ensure that "make
test" works in each subdirectory (even if it does not actually run any
applications). The top-level demo makefile now works.

The makefiles are not make-agnostic. e.g. they use the variable $(RM)
in "clean" recipes, which is defined in gnu-make but may not be
defined in others.

Part of #17806

Testing:

$ cd demo
$ make test

Reviewed-by: Paul Dale <pauli@openssl.org>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/22698)

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