examples: Add an option to claim the package as GNU/non-GNU.
This mirrors the Makefile.in.in and Makevars change from 2014-05-01.
Reported by David Shea at <https://savannah.gnu.org/bugs/?40520>.
* gettext-runtime/po/Makefile.in.in ($(DOMAIN).pot-update): Tweak whitespace.
* gettext-tools/examples/hello-*/po/Makefile.am (PACKAGE_GNU): New variable.
($(DOMAIN).pot-update): Don't search for "GNU packagename" if $(PACKAGE_GNU) is
set.
* gettext-tools/examples/hello-objc-gnustep/po/GNUmakefile: Likewise.
