po/Makefile.am: add some default values
po/Makefile.am: add some default values

This allows the makefile to be involved standalone, which allows
generating an updated .pot file without installing all of the
dependencies required to build cockpit.

It's now possible to run something like

  make -C /path/to/repo -f po/Makefile.am po/cockpit.pot

to get a freshly synced cockpit.pot.

martinpitt/cockpit,cockpit-project/cockpit,cockpit-project/cockpit,mvollmer/cockpit,mvollmer/cockpit,mvollmer/cockpit,martinpitt/cockpit,martinpitt/cockpit,cockpit-project/cockpit,martinpitt/cockpit,cockpit-project/cockpit,martinpitt/cockpit,mvollmer/cockpit,mvollmer/cockpit,cockpit-project/cockpit