[CMake] Overriding read-only project's CMakeLists.txt

[Kenneth Porter]

Where can I put build instructions that are build-host-wide, not 
project-wide?

I'm relatively new to cmake. I want to "inject" some code before the 
project's own script, and I'd prefer not to modify the project (since then 
I'd have to maintain the patch).

I want to set the CMAKE_<target>_PREFIX variables based on the version of 
Visual Studio used and whether it's a debug or release build. (I've been 
given code to do this on the VTK list, but I don't see how to use it apart 
from editing VTK's file.)