pthread-h: Respect --enable-threads={posix|windows} option on mingw.
* m4/pthread_h.m4 (gl_PTHREAD_H): Require gl_THREADLIB. Set
HAVE_PTHREAD_H if gl_threads_api is 'windows'.
(LIB_PTHREAD): Rely on $LIBMULTITHREAD from threadlib.m4.
* modules/pthread (Link): Change to $(LIBMULTITHREAD).
* modules/pthread-h (Depends-on): Add threadlib.
(Link): Change to $(LIBTHREAD).
* modules/pthread-h-c++-tests (test_pthread_c___LDADD): Use
$(LIBMULTITHREAD) instead of $(LIB_PTHREAD).