[CMake] Feature wish - constrained string values for configuration options

[Robert Haines]

> I would like to point at my feature wish:
> http://www.vtk.org/Bug/view.php?id=7313
>
> Opinions?

I was looking for this functionality the other day, I think it's a  
good idea. I had thought that it was already in there for the  
CMAKE_BUILD_TYPE option that you always get but that seems to be  
implemented in a different way.

So +1 from me. Is there anyway I can reflect that in the bug tracker?

Rob