(HAVE_PTHREAD, LIB_PTHREAD): Port better to FreeBSD,
by also checking for pthread_create, pthread_self, pthread_sigmask.
Tighten the test for pthread_atfork while we're at it.
(HAVE_PTHREAD, LIB_PTHREAD): Port better to FreeBSD,
by also checking for pthread_create, pthread_self, pthread_sigmask.
Tighten the test for pthread_atfork while we're at it.