[CMake] HOWTO: Generate Use*.cmake files for installation

[kent williams]

I need to do what ITK does -- generate one set of *Config.cmake and
Use*.cmake files for the build tree, so it can be used in place, and a
second set to be installed when 'make install' is run.

Looking through the Insight CMakeLists.txt files I'm not seeing how or
where the files intended for the installed package are generated.

What am I missing?