setlocale: Improve support for locales not supported by libc.
Reported by Dapeng Gao <peter@dpgao.cc> at
<https://savannah.gnu.org/bugs/?54479>.
* gettext-runtime/intl/setlocale.c: Include <stdio.h>.
(libintl_setlocale): Use a more error-tolerant strategy when the locale to be
set is not supported by libc: Emit warnings instead of failing.
