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/default.nix

@@ -10,7 +10,7 @@
 # system, e.g., cygwin and mingw builds on i686-cygwin.  Most people
 # can ignore it.
 
-{system, stdenvType ? system, allPackages ? import ../.., platform}:
+{ system, stdenvType ? system, allPackages ? import ../.., platform, config }:
 
 assert system != "i686-cygwin" -> system == stdenvType;
 
@@ -24,7 +24,7 @@ rec {
   # be used with care, since many Nix packages will not build properly
   # with it (e.g., because they require GNU Make).
   stdenvNative = (import ./native {
-    inherit system allPackages;
+    inherit system allPackages config;
   }).stdenv;
 
   stdenvNativePkgs = allPackages {
@@ -35,13 +35,14 @@ rec {
 
   # The Nix build environment.
   stdenvNix = import ./nix {
+    inherit config;
     stdenv = stdenvNative;
     pkgs = stdenvNativePkgs;
   };
 
 
   # Linux standard environment.
-  stdenvLinux = (import ./linux {inherit system allPackages platform;}).stdenvLinux;
+  stdenvLinux = (import ./linux { inherit system allPackages platform config;}).stdenvLinux;
 
 
   # MinGW/MSYS standard environment.