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/linux/default.nix

@@ -7,7 +7,7 @@
 # The function defaults are for easy testing.
 { system ? builtins.currentSystem
 , allPackages ? import ../../top-level/all-packages.nix
-, platform ? null }:
+, platform ? null, config }:
 
 rec {
 
@@ -81,7 +81,7 @@ rec {
     {gcc, extraAttrs ? {}, overrides ? (pkgs: {}), extraPath ? [], fetchurl}:
 
     import ../generic {
-      inherit system;
+      inherit system config;
       name = "stdenv-linux-boot";
       preHook =
         ''
@@ -261,7 +261,7 @@ rec {
   #     dependency (`nix-store -qR') on bootstrapTools or the
   #     first binutils built.
   stdenvLinux = import ../generic rec {
-    inherit system;
+    inherit system config;
     
     preHook = commonPreHook;