[CMake] Confusion with include() relativity

[Robert Dailey]

I have an interesting structure for my code & build scripts:

root/
  source/
    CMakeLists.txt
    build/
      CMakeLists.txt
  cmake/
    common.cmake

My root CMakeLists.txt is actually in 'root/source/build", and my
common scripts are in "root/cmake".