localename: Fine-tune support for per-thread locales on Solaris 11.4.

localename: Fine-tune support for per-thread locales on Solaris 11.4.

* lib/localename-table.h: New file, extracted from lib/localename.c.
* lib/localename-table.c: Likewise.
* lib/localename.c: Include localename-table.h.
(get_locale_t_name, newlocale, duplocale, freelocale): Invoke
locale_hash_function instead of pointer_hash.
* modules/localename (Files): Add lib/localename-table.h,
lib/localename-table.c.
(lib_SOURCES): Add localename-table.c.
* m4/intlsolaris.m4 (gt_INTL_SOLARIS): Require AC_CANONICAL_HOST. Test
for Solaris 11.4 locale system only on Solaris. Test for it
independently whether getlocalename_l exists.
* m4/intl.m4 (gt_INTL_SUBDIR_CORE): Don't test for 'uselocale' and
'getlocalename_l'. Instead, invoke gt_INTL_SOLARIS. Set
HAVE_NAMELESS_LOCALES.
* modules/gettext (Files): Add m4/intlsolaris.m4.