[CMake] How to declare global variables

[Robert Dailey]

Currently I'm having a bit of trouble making variables defined in the
following directory:

C:\a\b\c\d\e\CMakeLists.txt

Available to another script in a distant directory, located here:

C:\a\b\x\y\z\CMakeLists.txt

I've tried using PARENT_SCOPE, but this doesn't declare it "high"
enough. The only possible way I've been able to make this work is by
making it an internal cache variable. However, I do not want to
persist the contents of the variable. Do I have any other options?