See #490 discussion.
This reverts commit 1278859d31, reversing
changes made to 0c020c98f9.
Conflicts:
pkgs/desktops/xfce/core/xfce4-session.nix (take master)
pkgs/lib/misc.nix (auto)
Also updated weston to 1.0.5, and got rid of the patch that's no longer needed.
They fixed it upstream even if it was actually caused by our pkg-config patch.