msgfmt: Supersede, not overwrite, the output file of type '.mo'.
* autogen.sh (GNULIB_MODULES_TOOLS_FOR_SRC): Add 'supersede'.
* gettext-tools/src/write-mo.c: Include supersede.h.
(msgdomain_write_mo): When writing to a file, use fopen_supersede and
fwriteerror_supersede.
* NEWS: Mention the change.
