Fix Vtk output in benchmark tests.
Include global rules to Makefile.am in cmake subfolder.

This fixes broken "make doc", "make headercheck" and so on.
[Misc] Integrate some fixes by Christoph

* 'master' of https://github.com/duneprojecttest/dune-pdelab:
  Include global rules to Makefile.am in cmake subfolder.
  Fix Vtk output in benchmark tests.