config: fix XDG basedir compliance

quoth the spec:

If $XDG_DATA_DIRS is either not set or empty, a value equal to
/usr/local/share/:/usr/share/ should be used.

If $XDG_CONFIG_DIRS is either not set or empty, a value equal to
/etc/xdg should be used.

https://bugzilla.gnome.org/show_bug.cgi?id=720118