> Mildly annoying that if you don't specify --HEAD that it will still download and extract the stable archive.

I guess you could probably write a `Requirement` for this that would check for a HEAD build, only be evaluated when qt is being built, and error out prior to staging the tarball. e.g., something like

```ruby
class HeadOnly < Requirement
  fatal true
  def satisfied?
    MacOS.version < :mavericks || build.head?
  end
end