Replace VTKConfig.cmake guard variable with VTK target

Rather than using a variable set in VTKConfig.cmake use a VTK target
vtkCommonCore to prevent multiple loads of the VTK targets. This is
needed in the case of ParaView that also defines VTK targets in its
config file.

Change-Id: Idfc9a9087475a4d1d074647e58454cc693f3f6e2