pthread_sigmask: Fix link requirements on OSF/1 5.1 and with pth.

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).