Remove gccStdInc

This reverts d927da8d. Having a copy
of gcc-wrapper/setup-hook.sh is bad for maintainability - it had
already started to diverge. Also, gccStdInc gave a nix-env conflict
with the standard gcc. And it wasn't actually used in Nixpkgs.

Instead, if you really need to change "-isystem" to "-I", you can now
set ccIncludeFlag to "-I".