Remove gccStdInc

This reverts d927da8dae. 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".

pkgs/build-support/cc-wrapper/setup-hook.sh

@@ -2,7 +2,7 @@ export NIX_CC=@out@
 
 addCVars () {
     if [ -d $1/include ]; then
-        export NIX_CFLAGS_COMPILE+=" -isystem $1/include"
+        export NIX_CFLAGS_COMPILE+=" ${ccIncludeFlag:--isystem} $1/include"
     fi
 
     if [ -d $1/lib64 -a ! -L $1/lib64 ]; then