pthread_sigmask: Make declaration C++ safe.

pthread_sigmask: Make declaration C++ safe.

* lib/signal.in.h: In two special conditions, just do an #include_next.
(pthread_sigmask): Test HAVE_PTHREAD_SIGMASK, not REPLACE_PTHREAD_MASK.
Invoke _GL_CXXALIAS_SYS and _GL_CXXALIASWARN.
* m4/signal_h.m4 (gl_SIGNAL_H_DEFAULTS): Initialize
HAVE_PTHREAD_SIGMASK, not REPLACE_PTHREAD_MASK.
* m4/pthread_sigmask.m4 (gl_PTHREAD_SIGMASK): Set HAVE_PTHREAD_SIGMASK,
not REPLACE_PTHREAD_MASK.
* modules/signal (Makefile.am): Substitute HAVE_PTHREAD_SIGMASK,
not REPLACE_PTHREAD_MASK.
* modules/pthread_sigmask (Depends-on, configure.ac): Update condition.