pango on darwin: attempt to fix build

Supply older flex (new doesn't build), and disable introspection.
llvm(Packages) on darwin: attempts to fix

- mark llvm34 as broken on darwin (so it doesn't install by default with nix-env)
- don't use our gcc for llvm_34 (might fix the build)
- switch also clang default to 3.3 on darwin (llvm was before)