* m4/threads_h.m4 (gl_THREADS_H): Test against AIX 7 thrd_join bug. Set
BROKEN_THRD_JOIN.
(gl_THREADS_H_DEFAULTS): Initialize BROKEN_THRD_JOIN.
* m4/thrd.m4 (gl_FUNC_THRD_JOIN): Set REPLACE_THRD_* to 1 also if
BROKEN_THRD_JOIN is 1. Define BROKEN_THRD_START_T_OR_JOIN instead of
BROKEN_THRD_START_T.
* modules/threads-h (Makefile.am): Substitute BROKEN_THRD_JOIN.
* lib/threads.in.h (rpl_thrd_t, thrd_t): Define also if BROKEN_THRD_JOIN
is 1.
* lib/thrd.c: Test BROKEN_THRD_START_T_OR_JOIN instead of
BROKEN_THRD_START_T.
* doc/posix-functions/thrd_join.texi: Update.
* doc/posix-functions/thrd_exit.texi: Likewise.