iswxdigit: Work around OpenBSD, Android, Haiku bug. * m4/iswxdigit.m4 (gl_FUNC_ISWXDIGIT): Require gt_LOCALE_EN_UTF8 instead of gt_LOCALE_FR_UTF8. Use LOCALE_EN_UTF8 instead of LOCALE_FR_UTF8. Update cross-compilation guess. * modules/iswxdigit (Files): Add m4/locale-en.m4. Remove m4/locale-fr.m4. * doc/posix-functions/iswxdigit.texi: Update list of platforms.