COMP: Fix VTK ExternalProject build with Ninja generator.
This fixes the passing of the build command, e.g. "ninja", when not using the 'Unix Makefile' generator for the "make" command on unix, to the VTK external build.
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This fixes the passing of the build command, e.g. "ninja", when not using the 'Unix Makefile' generator for the "make" command on unix, to the VTK external build.