Fix tarball build

pkgs/development/compilers/go/1.3.nix

@@ -1,5 +1,7 @@
 { stdenv, lib, fetchurl, bison, glibc, bash, coreutils, makeWrapper, tzdata, iana_etc }:
 
+assert stdenv.gcc.gcc != null;
+
 let
   loader386 = "${glibc}/lib/ld-linux.so.2";
   loaderAmd64 = "${glibc}/lib/ld-linux-x86-64.so.2";

pkgs/development/compilers/rustc/0.11.nix

@@ -1,5 +1,7 @@
 {stdenv, fetchurl, which, file, perl, curl, python27, makeWrapper}:
 
+assert stdenv.gcc.gcc != null;
+
 /* Rust's build process has a few quirks :
 
 - It requires some patched in llvm that haven't landed upstream, so it

pkgs/development/compilers/rustc/head.nix

@@ -1,5 +1,7 @@
 {stdenv, fetchurl, fetchgit, which, file, perl, curl, python27, makeWrapper}:
 
+assert stdenv.gcc.gcc != null;
+
 /* Rust's build process has a few quirks :
 
 - It requires some patched in llvm that haven't landed upstream, so it