po: Prefer intltool's po/Makefile.in.in to gettext's

We currently use intltool to translate ibus-setup-m17n.desktop.
However, the po/Makefile.in.in file created by autopoint is no longer
compatible with the latest intltool.  On the other hand, the latest
gettext got support for .desktop file, but I expect some more time
until it will be widely adopted.  Let's rely on intltool for now.

BUG=https://bugzilla.redhat.com/show_bug.cgi?id=1078169

Review URL: https://codereview.appspot.com/143130043