Prepare for locale dependent tests on mingw.

Prepare for locale dependent tests on mingw.

* m4/locale-ar.m4 (gt_LOCALE_AR): On native Windows, don't try "ar"
because it has the wrong locale encoding.
* m4/locale-fr.m4 (gt_LOCALE_FR): On native Windows, try
French_France.1252 instead of "fr".
(gt_LOCALE_FR_UTF8): On native Windows, try French_France.65001.
* m4/locale-ja.m4 (gt_LOCALE_JA): On native Windows, don't try "ja"
because it has the wrong locale encoding.
* m4/locale-tr.m4 (gt_LOCALE_TR_UTF8): Require AC_CANONICAL_HOST. On
native Windows, try Turkish_Turkey.65001.
* m4/locale-zh.m4 (gt_LOCALE_ZH_CN): On native Windows, try
Chinese_China.54936.