localename: Fix link error with duplocale on AIX 7.2 (regr. 2019-12-18).

localename: Fix link error with duplocale on AIX 7.2 (regr. 2019-12-18).

* m4/locale_h.m4 (gl_LOCALE_H_DEFAULTS): Initialize
LOCALENAME_ENHANCE_LOCALE_FUNCS.
* m4/intl-thread-locale.m4 (gt_FUNC_USELOCALE):
Set gt_working_uselocale.
(gt_INTL_THREAD_LOCALE_NAME): Set gt_fake_locales, gt_good_uselocale,
gt_localename_enhances_locale_funcs, LOCALENAME_ENHANCE_LOCALE_FUNCS.
Define HAVE_GOOD_USELOCALE, LOCALENAME_ENHANCE_LOCALE_FUNCS.
* m4/localename.m4 (gl_LOCALENAME): Test
gt_localename_enhances_locale_funcs instead of gt_nameless_locales.
* modules/locale (Makefile.am): Substitute
LOCALENAME_ENHANCE_LOCALE_FUNCS.
* lib/locale.in.h (newlocale, duplocale, freelocale): Don't override if
module 'localename' is in use but LOCALENAME_ENHANCE_LOCALE_FUNCS is 0.
* lib/localename.c (HAVE_GOOD_USELOCALE): Don't define here.
(get_locale_t_name, newlocale, duplocale, freelocale): Define if
LOCALENAME_ENHANCE_LOCALE_FUNCS.