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/generic/setup.sh

@@ -869,4 +869,10 @@ genericBuild() {
 runHook postHook
 
 
+# Execute the global user hook (defined through the Nixpkgs
+# configuration option ‘stdenv.userHook’).  This can be used to set
+# global compiler optimisation flags, for instance.
+runHook userHook
+
+
 dumpVars