BUILD: fix GLIB_GNU_GETTEXT to cope without po/POTFILES.in
BUILD: fix GLIB_GNU_GETTEXT to cope without po/POTFILES.in

tim-janik/beast,swesterfeld/beast,GNOME/beast,tim-janik/beast,swesterfeld/beast,GNOME/beast,swesterfeld/beast,GNOME/beast,GNOME/beast,swesterfeld/beast,swesterfeld/beast,swesterfeld/beast,tim-janik/beast,tim-janik/beast,swesterfeld/beast,tim-janik/beast,GNOME/beast,tim-janik/beast,GNOME/beast,tim-janik/beast,GNOME/beast