Build kit for Python modules
Building of the "kit" for VTK Python modules is required for Slicer. This merge request backports some changes from https://github.com/Slicer/VTK to upstream. The specific commits are:
https://github.com/Slicer/VTK/compare/61cdd20...61a46c
Several additional commits fix Python tests failures and a linker error when building with the OpenGL2 backend (http://www.vtk.org/Bug/view.php?id=15255).