these 2 derivations will be built:
  /nix/store/davf7qqbkc22d4gqb6w6cqi9p4pqas5a-libxl-lin-3.9.4.3.tar.gz.drv
  /nix/store/13lfirm7rq9y3adac9cnxiafbglzmvrq-libxl-3.9.4.3.drv