new Go support function 'buildGoPackage'

add a setupHook to the Go compiler that builds a GOPATH
new buildGoPackage function that overides the phases of stdenv

pkgs/development/compilers/go/1.3.nix

@@ -81,6 +81,7 @@ stdenv.mkDerivation {
   installPhase = ''
     export CC=cc
     mkdir -p "$out/bin"
+    unset GOPATH
     export GOROOT="$(pwd)/"
     export GOBIN="$out/bin"
     export PATH="$GOBIN:$PATH"
@@ -99,6 +100,8 @@ stdenv.mkDerivation {
     cp ./misc/emacs/* $out/share/emacs/site-lisp/
   '';
 
+  setupHook = ./setup-hook.sh;
+
   meta = {
     homepage = http://golang.org/;
     description = "The Go Programming language";

pkgs/development/compilers/go/1.4.nix

@@ -95,6 +95,8 @@ stdenv.mkDerivation {
     go test -v    $TOOL_ROOT/cover $TOOL_ROOT/vet # $TOOL_ROOT/godoc
   '';
 
+  setupHook = ./setup-hook.sh;
+
   meta = {
     homepage = http://golang.org/;
     description = "The Go Programming language";

pkgs/development/compilers/go/setup-hook.sh

@@ -0,0 +1,5 @@
+addToGoPath() {
+    addToSearchPath GOPATH $1/share/go
+}
+
+envHooks=(${envHooks[@]} addToGoPath)