mbtowc: Fix compilation error on Android 4.3. * lib/stdlib.in.h (mbtowc): Test also HAVE_MBTOWC. * m4/mbtowc.m4 (gl_FUNC_MBTOWC): Test whether mbtowc exists. Set HAVE_MBTOWC. * m4/stdlib_h.m4 (gl_STDLIB_H): Test whether mbtowc is declared. (gl_STDLIB_H_DEFAULTS): Initialize HAVE_MBTOWC. * modules/stdlib (Makefile.am): Substitute HAVE_MBTOWC. * modules/mbtowc (Depends-on, configure.ac): Test also HAVE_MBTOWC. * doc/posix-functions/mbtowc.texi: Mention the change.