setlocale: Improve locale handling on macOS 10.12 or newer. * lib/setlocale.c: Include header files for CoreFoundation. Declare gl_locale_name_canonicalize. (libintl_setlocale): Try harder to set a locale for categories LC_CTYPE and LC_MESSAGES. * m4/setlocale.m4 (gl_PREREQ_SETLOCALE): Add comment.