libintl: Fix language preferences on macOS 10.12 or newer.
Reported by Kristian Rietveld <kris@loopnest.org> at
<https://savannah.gnu.org/bugs/?49560>.
* gettext-runtime/intl/langprefs.c (_nl_language_preferences_default): Handle
preferences elements of the form "ll-CC" in a reasonable way.
* NEWS: Mention the change.
