make it possible to use setup.py without having an installed dune-common...

make it possible to use setup.py without having an installed dune-common python package. This requires having the PYTHONPATH set as it is now done in dune-common/bin/dunepackaging.py

[bugfix] previous commit

[bugfix] previous commit

set the correct 'prefix' in the pc files if SKBUILD is set, i.e.,
a wheel is being build using scikit-build