* m4/mbsrtowcs.m4 (gl_FUNC_MBSRTOWCS): Invoke gl_MBRTOWC_C_LOCALE. If
mbrtowc is buggy in the C locale, override also mbsrtowcs.
* modules/mbsrtowcs (Files): Add m4/mbrtowc.m4.
* tests/test-mbsrtowcs.c (main): Add a test of the C locale, based on
tests/test-mbrtowc.c.
* tests/test-mbsrtowcs5.sh: New file, based on tests/test-mbrtowc5.sh.
* modules/mbsrtowcs-tests (Files): Add it.
(Makefile.am): Test it.
* doc/posix-functions/mbsrtowcs.texi: Mention the C locale behaviour
bug.