gobject-introspection: Deal with $outputLib

Once #7701 gets merged, we have another environment variable called
$outputLib, which then points to another environment variable which is
the final library output.

This was brought up in discussion with @lethalman and @vcunat in:

https://github.com/NixOS/nixpkgs/pull/12558#discussion_r50599813



The closure-size branch is not yet merged into master, so this is only
a preparation and we're still falling back to $out and $lib whenever
$outputLib isn't available.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>