* lib/vasnprintf.c (VASNPRINTF): If NEED_WPRINTF_DIRECTIVE_C is set,
implement the 'c' directive here.
* m4/vasnprintf.m4 (gl_PREREQ_VASNWPRINTF): Invoke gl_MBRTOWC_C_LOCALE.
If mbrtowc is buggy in the C locale, define NEED_WPRINTF_DIRECTIVE_C.
* modules/vasnwprintf (Files): Add m4/mbrtowc.m4.
* tests/test-vasnwprintf-posix.c (test_function): Add tests of %s and %c
in the C locale.
* doc/posix-functions/fwprintf.texi: Mention the C locale behaviour bug.
* doc/posix-functions/swprintf.texi: Likewise.
* doc/posix-functions/vfwprintf.texi: Likewise.
* doc/posix-functions/vswprintf.texi: Likewise.
* doc/posix-functions/vwprintf.texi: Likewise.
* doc/posix-functions/wprintf.texi: Likewise.