[RFC] Enable cpack variable overriding via cmake cache

Hi!

I have a patch which I’ve uploaded which I would love to see make it downstream at some point:

https://reviews.llvm.org/D107532

I’m not totally sure who to include as a reviewer, I just went with git-blame and CODE_OWNERS for hints. If anyone qualified wants to take a look (especially cmake experts), it’s a pretty small change that I’ve been using in my local builds for some time.

Appreciate whatever pointers and hints I can get from the community on how to get this (or something like it) into a release.

Thanks!
-Jon Reeves