[polly][cmake] Use `GNUInstallDirs` to support custom installation dirs

I am breaking apart D99484 so the cause of build failures is easier to
understand.

Reviewed By: Meinersbur

Differential Revision: https://reviews.llvm.org/D117541

GitOrigin-RevId: d3b756c51c1b42f653b8abf2b2e83a2321befb9d
3 files changed