pthread-h: Fix link errors with --enable-threads=isoc on AIX.

* m4/threadlib.m4 (gl_ANYTHREADLIB_EARLY): New macro, extracted from
gl_THREADLIB_EARLY_BODY.
(gl_THREADLIB_EARLY_BODY): Invoke it.
(gl_PTHREADLIB_BODY): New macro, extracted from gl_THREADLIB_BODY.
(gl_THREADLIB_BODY): Invoke it.
(gl_PTHREADLIB): New macro.
* m4/pthread_h.m4 (gl_PTHREAD_H): Require gl_PTHREADLIB. Require
gl_THREADLIB and test $gl_threads_api only if module 'threadlib' is
present. Define LIB_PTHREAD using LIBPMULTITHREAD, not LIBMULTITHREAD.
* modules/pthread-h (Files): Add threadlib.m4.
(Depends-on): Remove threadlib.
(configure.ac-early): Invoke gl_ANYTHREADLIB_EARLY. Don't set _REENTRANT
and _THREAD_SAFE here.
(Link): Use LIBPTHREAD, not LIBTHREAD.
* modules/pthread-thread (Link): Use LIBPMULTITHREAD, not
LIBMULTITHREAD.
* modules/pthread-once (Link): Likewise.
* modules/pthread-mutex (Link): Likewise.
* modules/pthread-rwlock (Link): Likewise.
* modules/pthread-cond (Link): Likewise.
* modules/pthread-tss (Link): Likewise.
* modules/pthread-spin (Link): Likewise.
* modules/pthread (Link): Likewise.
* modules/pthread-h-c++-tests (test_pthread_c___LDADD): Likewise.
* modules/pthread-thread-tests (test_pthread_thread_LDADD): Likewise.
* modules/pthread-once-tests (test_pthread_once1_LDADD,
test_pthread_once2_LDADD): Likewise.
* modules/pthread-mutex-tests (test_pthread_mutex_LDADD): Likewise.
* modules/pthread-rwlock-tests (test_pthread_rwlock_LDADD): Likewise.
* modules/pthread-cond-tests (test_pthread_cond_LDADD): Likewise.
* modules/pthread-tss-tests (test_pthread_tss_LDADD): Likewise.

modules/pthread-mutex-tests

@@ -11,4 +11,4 @@ configure.ac:
 Makefile.am:
 TESTS += test-pthread-mutex
 check_PROGRAMS += test-pthread-mutex
-test_pthread_mutex_LDADD = $(LDADD) @LIBMULTITHREAD@ @YIELD_LIB@
+test_pthread_mutex_LDADD = $(LDADD) @LIBPMULTITHREAD@ @YIELD_LIB@