[Cmake] Building in two separate trees

[Amitha Perera]

Hi all

I know that CMake can be used to build in two separate trees using the
LoadCache command. For example, the vxl tree is build in one
directory, and our internal libraries are built somewhere else. The
relevant settings, such as dependencies, are extracted from the vxl
tree using LoadCache.

Now, LoadCache brings in pretty much everything in the cache, and
marks them unmodifiable in the new tree. This is not appropriate for
some entries, such as CMAKE_CXX_FLAGS and CMAKE_VERBOSE_MAKEFILE. Is
there a better way than excluding each variable? I recall Brad
mentioned a new style of doing things in vtk.

Thanks,
Amitha.