Python dependency should be optional
Building against a VTK built without python fails with:
CMake Warning at CMakeLists.txt:19 (find_package):
Found package configuration file:
/home/glow/work/vtk/vtk_build/vtk-config.cmake
but it set VTK_FOUND to FALSE so package "VTK" is considered to be NOT
FOUND. Reason given by package:
Could not find the VTK package with the following required components:
Python.
It should be supported to build without it.