Not every package that needs xcbuild will want to use its build phase.
I have moved the xcbuild setup hook to the new attribute xcbuildHook.
This means that dontUseXcbuild is no longer needed. If you just need
to call xcbuild on its own you can just refer to xcbuild.
This includes adding a new xcbuild-based libutil build to test the waters a bit there.
We'll need to get xcbuild into the stdenv bootstrap before we can make the main build,
but it's nice to see that it can work.
pkill isn't building because of some missing headers:
- xpc/xpc.h
- os/base_private.h
- _simple.h
They are all available somewhere but not set up correctly in the Darwin
stdenv.
TODO: add pkill back in!
