Add an option ‘stdenv.userHook’ to set a global stdenv setup hook

This allows various applications.  It allows users to set global
optimisation flags, e.g.

  stdenv.userHook = ''NIX_CFLAGS_COMPILE+=" -funroll-loops"'';

But the impetus is as an alternative to issue #229, allowing impure
stdenv setup for people who want to use distcc:

  stdenv.userHook = "source /my/impure/setup-script.sh";

This is probably a bad idea, but at least now it's a bad idea in
people's configuration and not in Nixpkgs. :-)

pkgs/stdenv/native/default.nix

@@ -1,4 +1,4 @@
-{ system, allPackages ? import ../../.. }:
+{ system, allPackages ? import ../../.., config }:
 
 rec {
 
@@ -98,7 +98,7 @@ rec {
 
       fetchurlBoot = fetchurl;
 
-      inherit system shell gcc overrides;
+      inherit system shell gcc overrides config;
     };