xcbuild: add wrapper

Also updates xcbuild version.

This changes the raw string expressions into nix expressions that are
then converted into json by builtins.toJSON. Then, converted to Plist
XML by Apple's plutil. Sadly, xcbuild does not support using raw JSON
but Apple's plutil does so we just convert the file from JSON to XML
using Apple's plutil. The result is not ideal but it looks like all OS X
systems have working plutil's.

- set mac version to 10.10
- add setup hook.