MacOS: Add fallback implementation for POSIX semaphore API.
* configure.ac [*-apple-darwin*] (HAVE_NO_POSIX_SEMAPHORE): New. * src/npth.c [HAVE_NO_POSIX_SEMAPHORE] (sem_init, sem_post, sem_wait): New.
- GnuPG-bug-id: T7057
- Signed-off-by: NIIBE Yutaka <gniibe@fsij.org>