installing numpy on darwin with UCS4 with nix -- the missing link

default.nix

# see https://github.com/NixOS/nixpkgs/blob/master/pkgs/os-specific/darwin/apple-sdk/frameworks.nix
let
  pkgs = import <nixpkgs> {};
  # MONEY LINE 1/2
  frameworks = pkgs.darwin.apple_sdk.frameworks;
  stdenv = pkgs.stdenv;
in stdenv.mkDerivation rec {
  name = "env";
  env = pkgs.buildEnv { name = name; paths = buildInputs; };
  buildInputs = [
    # NOTE: do NOT include pkgs.python27Packages.pip;
    # it will interfere with the virtualenv pip
    pkgs.python27Packages.virtualenv

    # MONEY LINE 2/2
    # this provides Accelerate, needed for numpy build
    frameworks.CoreGraphics
  ];
  shellHook = ''
    if [ ! -e $PWD/venv ]; then
        virtualenv venv
    fi
    source venv/bin/activate
  '';
}