* lib/stdlib.in.h (mbstowcs): New declaration.
* lib/mbstowcs.c: New file, based on lib/mbstoc32s.c.
* m4/mbstowcs.m4: New file.
* m4/stdlib_h.m4 (gl_STDLIB_H): Test whether mbstowcs is declared.
(gl_STDLIB_H_REQUIRE_DEFAULTS): Initialize GNULIB_MBSTOWCS.
(gl_STDLIB_H_DEFAULTS): Initialize REPLACE_MBSTOWCS.
* modules/stdlib (Makefile.am): Substitute GNULIB_MBSTOWCS,
REPLACE_MBSTOWCS.
* modules/mbstowcs: New file.
* tests/test-stdlib-c++.cc (mbstowcs): Check signature.
* doc/posix-functions/mbstowcs.texi: Mention the C locale behaviour bug
and the new module.