Makefile.in.in: Add an option to claim the package as GNU/non-GNU
Reporeted by David Shea at:
<https://savannah.gnu.org/bugs/?40520>.
* Makevars (PACKAGE_GNU): New variable.
* Makefile.in.in ($(DOMAIN).pot-update): Don't search for "GNU
packagename" if $(PACKAGE_GNU) is set.
