Issue description

I'm working towards adding Darwin support for gstreamer. But I cannot compile gst-plugins-bad because of Apple SDK/framework build errors. Compiler output can be read here https://gist.github.com/Kaali/0b9e8fecd5a35989e9ae968d059ffa96

Steps to reproduce

Apply patch from here: https://gist.github.com/Kaali/483236da1407c750f305533fe8e25983

Run nix-build -A mopidy. mobidy is the final application I'm working towards, but I think it's not really important for this issue, just that something requires gst-plugins-bad.

Technical details

 - system: `"x86_64-darwin"`
 - host os: `Darwin 17.4.0, macOS 10.13.3`
 - multi-user?: `yes`
 - sandbox: `no`
 - version: `nix-env (Nix) 1.11.16`
 - channels(kaali): `""`
 - channels(root): `"nixpkgs-18.03pre127302.e860b651d6e"`
 - nixpkgs: `/nix/var/nix/profiles/per-user/root/channels/nixpkgs`