python: add an option to disable vtkpython

ParaView doesn't care about vtkpython and when a static python build is
used, plumbing the required extra libraries down to vtkpython would also
require a magic variable for ParaView to set.

Change-Id: Iff6db1f3a6e897b92ddd215c293ae7d7281602af
testing: check for pvtkpython before using it

Change-Id: I4fcd40fdfb1b714c26486562d49715015445ea8f
vtkpython: don't install if not enabled

Change-Id: I5dda10540bde3dbdb7a4edfd3bb28f515cc8dc87
Merge topic 'vtkpython-option' into master

3a061664 vtkpython: don't install if not enabled
3e937dff testing: check for pvtkpython before using it
987f1f40 python: add an option to disable vtkpython