pthread_sigmask: Fix compilation error with --enable-threads=windows.
Reported by Tim Rühsen in
<https://lists.gnu.org/archive/html/bug-gnulib/2018-01/msg00018.html>
and Michele Locati in
<https://lists.gnu.org/archive/html/bug-gettext/2019-04/msg00057.html>.
* m4/pthread_sigmask.m4 (gl_FUNC_PTHREAD_SIGMASK): Revert change from
2015-06-01. Test whether pthread_sigmask is a macro, regardless of
$LIBMULTITHREAD. Consider it regardless whether module 'threadlib' is
in use and regardless which threads API is chosen.