[vtkusers] VTK_MAKE_INSTANTIATOR2 and VTK_MAKE_INSTANTIATOR3

[Sean McInerney]

Hi all (especially VTK build system architects),

Can anyone describe the cmake code to properly import VTK's 
VTK_MAKE_INSTANTIATOR2 (4.2 <= VTK < 4.5) and VTK_MAKE_INSTANTIATOR3 
(4.5 <= VTK) for use in external projects.

For me, use of VTK_MAKE_INSTANTIATOR2 works just fine while 
VTK_MAKE_INSTANTIATOR3 does not ... and I have no idea why.

-Sean