pthread_sigmask: Fix link requirements on OSF/1 5.1 and with pth. * lib/signal.in.h (pthread_sigmask): Override if REPLACE_PTHREAD_SIGMASK is 1. * m4/signal_h.m4 (gl_SIGNAL_H_DEFAULTS): Initialize REPLACE_PTHREAD_SIGMASK. * m4/pthread_sigmask.m4 (gl_FUNC_PTHREAD_SIGMASK): Set also REPLACE_PTHREAD_SIGMASK. Set and substitute LIB_PTHREAD_SIGMASK. * modules/signal (Makefile.am): Substitute REPLACE_PTHREAD_SIGMASK. * modules/pthread_sigmask (Depends-on, configure.ac): Update condition. (Link): Set to $(LIB_PTHREAD_SIGMASK), not $(LIBMULTITHREAD).