Unconditionally disable threading, it is apparently causing some
edge cases to fail, and the previous version of gettext was unthreaded,
so we're not losing anything.
Note that if you've already rebuilt gettext to 0.16.1, you only need
to upgrade gettext, and not everything that depends on it.
Thanks to those that reported the issue, and both gnome@ and kde@ for
confirming that this change does not cause any issues there.