Matplotlib was not working in ParaView binaries.

PARAVIEW_ENABLE_MATPLOTLIB should not depend on whether matplotlib was
found at configure time. This is causing issues with superbuilds and
failing to enable matplotlib components.

Change-Id: I531ce579b1585840b293bdaf41008c7ba890c69e
Merge topic 'fix_matplot_lib' into next

9327c32a Matplotlib was not working in ParaView binaries.