New module 'wctomb'. * lib/stdlib.in.h (wctomb): New declaration. * lib/wctomb.c: New file. * lib/wctomb-impl.h: New file. * m4/wctomb.m4: New file. * m4/stdlib_h.m4 (gl_STDLIB_H_DEFAULTS): Initialize GNULIB_WCTOMB, REPLACE_WCTOMB. * modules/stdlib (Makefile.am): Substitute GNULIB_WCTOMB, REPLACE_WCTOMB. * modules/wctomb: New file. * tests/test-stdlib-c++.cc: Test signature of wctomb. * doc/posix-functions/wctomb.texi: Mention the new module. * modules/wctob (Depends-on): Add wctomb.