SDL_SemValue() always returns 0. That's because the underlying POSIX
sem_getvalue() is implemented as a noop in Mac OS X. Apart from that,
semaphores do work properly (at least according to test/testsem), so I'm not
sure if this is worth fixing at all.

--HG--
extra : convert_revision : svn%3Ac70aab31-4412-0410-b14c-859654838e24/trunk%401634