Python: improve cross-compilation

This changeset allows for cross-compilation of Python packages. Packages
built with buildPythonPackage are not allowed to refer to the build
machine. Executables that have shebangs will refer to the host.

pkgs/development/interpreters/python/cpython/2.7/default.nix

@@ -32,6 +32,9 @@ assert x11Support -> tcl != null
 with stdenv.lib;
 
 let
+
+  pythonForBuild = buildPackages.${"python${sourceVersion.major}${sourceVersion.minor}"};
+
   passthru = passthruFun rec {
     inherit self sourceVersion packageOverrides;
     implementation = "cpython";