Merge branch 'mac-purity' of git://github.com/copumpkin/nixpkgs

Let’s compile the Mac OS X SecurityTool ourselves

copumpkin:

This allows us to compile SecurityTool ourselves. There are several more
Apple opensource projects that can be compiled this way that I'll slowly
add.

Remaining sources of impurity:

Reference to absolute path to Xcode. This should be integrated with the
xcode derivation (and the iOS wrapper chain that exists under mobile
development) but it's not obvious how to do that yet.
Absolute reference to xcodebuild.
Adding this should make it possible for #3629 to work reasonably
cleanly.