po: pass the custom xgettext args through
Same as with the autotools build, see po/MakeVars
This makes translations work with meson now, in the usual fashion:
$ meson translation-build
$ ninja -C translation-build xkeyboard-config-pot
# Now you can optionally remove the build directory
$ rm -rf translation-build
$ cp po/xkeyboard-config.pot po/$lang.po
To update an existing translation, run:
$ meson translation-build
$ ninja -C translation-build xkeyboard-config-update-po
# Now you can optionally remove the build directory
$ rm -rf translation-build