* lib/uchar.in.h (mbrtoc32): New declaration.
* lib/mbrtoc32.c: New file, based on lib/mbrtowc.c.
* m4/mbrtoc32.m4: New file, based on m4/mbrtowc.m4.
* m4/uchar.m4 (gl_UCHAR_H): Test whether mbrtoc32 is declared.
(gl_UCHAR_H_DEFAULTS): Initialize GNULIB_MBRTOC32, HAVE_MBRTOC32,
REPLACE_MBRTOC32.
* modules/uchar (Makefile.am): Substitute GNULIB_MBRTOC32,
HAVE_MBRTOC32, REPLACE_MBRTOC32.
* modules/mbrtoc32: New file, based on modules/mbrtowc.
* tests/test-uchar-c++.cc (mbrtoc32): Verify the signature.
* modules/uchar-c++-tests (Makefile.am): Link test-uchar-c++ with
$(LIB_MBRTOWC).
* doc/posix-functions/mbrtoc32.texi: Document the new module.
* doc/posix-functions/mbrtowc.texi: Mention the new module.