Restore the versioning of the installed *.mo files

It seems slightly more elegant to use the GETTEXT_PACKAGE output
variable instead of hard coding the version suffix in po/Makevars.

Unfortunately, I couldn't find a way to not distribute the generated
po/Makevars, or a way to insert custom variables in Gettext's
Makevars. Failing that, since the value of GETTEXT_PACKAGE isn't going
to change as a result of configuring the build, we stop distcheck from
complaining about po/Makevars not getting removed after distclean by
using distcleancheck_listfiles.

Fallout from 37262520

https://bugzilla.gnome.org/show_bug.cgi?id=789010