A completely built dune-uggrid is rebuilt nevertheless

To reproduce:

1. Make new directory
2. In there, clone dune-common, dune-geometry, dune-grid, dune-uggrid 3a) dunecontrol all 3b) Result: Every module is built 4a) dunecontrol all 4b) Result: dune-uggrid is built again, even though it has been built in 3a)

Weirdly enough: 5a) dunecontrol all 5b) Result: This time, dune-uggrid is not rebuilt.