This seems to have been confusing people, using both xlibs and xorg, etc. - Avoided renaming local (and different) xlibs binding in gcc*. - Fixed cases where both xorg and xlibs were used. Hopefully everything still works as before.
Per https://github.com/NixOS/nixpkgs/pull/8422#issuecomment-113695261
Added systemd to buildEnv path because of new libudev dependency.