If $XDG_CONFIG_DIRS is already set, don't discard it

Some upstreams like LXQt will want to get configuration from locations that aren't /etc/xdg{,/xdg-$DESKTOP_SESSION}. This ensures that if $XDG_CONFIG_DIRS is already set, while it isn't put at the beginning of the variable (so correct settings are still set), it makes sure it's still included.

Not doing this is also against the upstream XDG spec, where it states: "If $XDG_CONFIG_DIRS is either not set or empty, a value equal to /etc/xdg should be used." The key word being "if" - if $XDG_CONFIG_DIRS is indeed set, we need to include that as well.

Should this be accepted, I don't have upload access to xorg in Ubuntu and would need sponsorship.