This moves the checks for POSIX semaphores to configure time. As well as
requiring <semaphore.h> and SEM_VALUE_MAX, we also require the
sem_timedwait function. That was only optional in POSIX 2001 (and is
absent on Darwin).

libstdc++-v3/ChangeLog:

	* acinclude.m4 (GLIBCXX_CHECK_GTHREADS): Check for
	* config.h.in: Regenerate.
	* configure: Regenerate.
	* include/bits/semaphore_base.h (_GLIBCXX_HAVE_POSIX_SEMAPHORE):
	Check autoconf macro instead of defining it here.