* m4/mbsnrtowcs.m4 (gl_FUNC_MBSNRTOWCS): Invoke gl_MBRTOWC_C_LOCALE. If
mbrtowc is buggy in the C locale, override also mbsnrtowcs.
* modules/mbsnrtowcs (Files): Add m4/mbrtowc.m4.
* tests/test-mbsnrtowcs.c (main): Add a test of the C locale, based on
tests/test-mbsrtowcs.c.
* tests/test-mbsnrtowcs5.sh: New file, based on tests/test-mbrtowc5.sh.
* modules/mbsnrtowcs-tests (Files): Add it.
(Makefile.am): Test it.
* doc/posix-functions/mbsnrtowcs.texi: Mention the C locale behaviour
bug.