COMP: Prevent Doxygen parsing of VTK CMake modules configured into build dir
This commit fixes a regression introduced in [1] where the VTK CMake modules have been updated to use sphinx commands provided by "sphinx-moderncmakedomain" extension.
This should allow updating VTK in ParaView by addressing warnings like the one reported here: https://open.cdash.org/viewBuildError.php?type=1&buildid=8755533
[1] Update & Consolidate documentation, and automatically publish at docs.vtk.org vtk/vtk!10164 (merged)
Edited by Jean-Christophe Fillion-Robin