[cmake][bugfix] Only call python macros if dune-python was found
dune-alugrid currently only sugggests dune-python, but uses CMake functionality provided by dune-python unconditionally. The patch guards those calls with the correct cmake variable.
dune-alugrid currently only sugggests dune-python, but uses CMake functionality provided by dune-python unconditionally. The patch guards those calls with the correct cmake variable.