build: Fix the build error in gettext-tools/examples/po.
Reported by Nick Alcock <nix@esperi.org.uk>
at <https://savannah.gnu.org/bugs/?56529>.
* gettext-tools/examples/po/Makefile.am (USE_BUILT_PROGS): New variable.
(hello-*.pot): Use it.
($(srcdir)/../hello-*/po/$(LL).po): Likewise.
* HACKING: Remove the workaround.
