intl: Simplify function prefixes.
* gettext-runtime/intl/configure.ac: Remove macro definitions of
_nl_locale_name_canonicalize, _nl_locale_name_from_win32_LANGID,
_nl_locale_name_from_win32_LCID, _nl_locale_name_thread_unsafe,
_nl_locale_name_posix, _nl_locale_name_default.
* gettext-runtime/intl/gettextP.h: Update declarations.
* gettext-runtime/intl/langprefs.c: Likewise.
(_nl_language_preferences_win32_mui, _nl_language_preferences_win32_ME,
_nl_language_preferences_win32_95, _nl_language_preferences_default): Use
gl_ prefixed symbols instead.
* gettext-runtime/intl/dcigettext.c (DCIGETTEXT, guess_category_value):
Likewise.
