kde4.kde_workspace: fixup build with gcc-6

Changed files
+2
pkgs
desktops
+2
pkgs/desktops/kde-4.14/kde-workspace.nix
···
--replace /usr/share/X11 ${xkeyboard_config}/etc/X11
'';
+
NIX_CFLAGS_COMPILE = [ "-fpermissive" ]; # gcc-6
+
enableParallelBuilding = false; # frequent problems on Hydra
meta = {