message: Copy.hash: new
  In order to depend on the post-process step.
diff: @@ -3125,6 +3125,14 @@ class Copy(Builder):
  def __str__(self):
  return 'copy of %s' % self.target
  + def hash(self):
  + try:
  + # Best effort: if we can hash this, let's do it. Otherwise no
  + # worse than before.
  + return self.__post_process.hash()
  + except AttributeError:
  + pass
  +
  class Install(Copy):
  """Builder to install files.
  