libstdc++: Ignore _GLIBCXX_USE_POSIX_SEMAPHORE if not supported [PR116992]

If _GLIBCXX_HAVE_POSIX_SEMAPHRE is undefined then users get an error
when defining _GLIBCXX_USE_POSIX_SEMAPHORE. We can just ignore it
instead (and warn them it's being ignored).

This fixes a testsuite failure on hppa64-hp-hpux11.11 (and probably some
other targets):

FAIL: 30_threads/semaphore/platform_try_acquire_for.cc  -std=gnu++20 (test for excess errors)
Excess errors:
semaphore:49: error: '__semaphore_impl' has not been declared

libstdc++-v3/ChangeLog:

	PR libstdc++/116992
	* include/bits/semaphore_base.h (_GLIBCXX_USE_POSIX_SEMAPHORE):
	Undefine and issue a warning if POSIX sem_t is not supported.
	* testsuite/30_threads/semaphore/platform_try_acquire_for.cc:
	Prune new warning.