Revert "Revert "Revert "Use `GNUInstallDirs` to support custom installation dirs. -- LLVM"""

This reverts commit 492de35d.

I tried to apply John's changes in 8d897ec9 that were expected to
fix his patch but that didn't work unfortunately.

Reverting this again to fix the macOS bots and leave him more time to
investigate the issue.