localename: Fix test failure on AIX 7.
Reported by Assaf Gordon in
<https://lists.gnu.org/archive/html/sed-devel/2018-12/msg00019.html>.
* m4/intl-thread-locale.m4 (gt_FUNC_USELOCALE): New macro.
(gt_INTL_THREAD_LOCALE_NAME): Invoke it. Test gt_cv_func_uselocale_works
instead of ac_cv_func_uselocale.
* lib/localename.c: Test HAVE_WORKING_USELOCALE instead of
HAVE_USELOCALE.
* lib/localename-table.h: Likewise.
* lib/localename-table.c: Likewise.
* tests/test-localename.c: Likewise.
* doc/posix-functions/uselocale.texi: Mention the AIX problem.