message: Fix drakefile prefix.
  Not sure why I did that.
diff: @@ -135,8 +135,6 @@ def configure(cxx_toolkit = None,
  if prefix is not False:
  prefix = drake.Path(prefix)
  - if not prefix.absolute():
  - prefix = drake.Path('..') / prefix
  if python3 is not False:
  if python_version is None:
  