Workaround intltool bug for make distcheck

intltool is not well maintained. The bug happens with a recent Automake
version. This can be reverted when gedit no longer uses intltool, or
when the intltool bug is fixed one day.

This is a bit ugly, but everything should not *always* be perfect all
the time.

Based on work from Sébastien Wilmet, at GEdit:

https://git.gnome.org/browse/gedit/commit/?id=10d29fb0fc6f3569f5c0bce140f3c9fb5ffd1b85