pthread_sigmask: Work around bug in single-threaded implementation.

pthread_sigmask: Work around bug in single-threaded implementation.

* m4/pthread_sigmask.m4 (gl_FUNC_PTHREAD_SIGMASK): Test for the
FreeBSD, HP-UX, Solaris bug.
(gl_PREREQ_PTHREAD_SIGMASK): New macro.
* lib/pthread_sigmask.c: Include <stddef.h>.
(pthread_sigmask): If HAVE_PTHREAD_SIGMASK, define as a wrapper around
the system's pthread_sigmask function.
* modules/pthread_sigmask (configure.ac): Invoke
gl_PREREQ_PTHREAD_SIGMASK.
* doc/posix-functions/pthread_sigmask.texi: Mention bug on FreeBSD,
HP-UX, Solaris.