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)
x-updates is supposed to merge after stdenv-updates, so let's test it
Conflicts:
pkgs/development/libraries/gtk+/2.x.nix (both updated, taking newer)
pkgs/development/libraries/mesa/default.nix (taking nativeBuildInputs)