Use CMake's system for IPO/LTO

AnyOldName3requested to merge
AnyOldName3/openmw:cmake-ipo into master

Resolves #5265 (closed). Supersedes https://github.com/OpenMW/openmw/pull/2686.

I've not tested this myself yet, I just rebased Eli2's attempt, fixed the merge conflicts, and tweaked it a little. CI should cover the non-LTO case, but it could do with people trying LTO.

Another thought - maybe we should enable LTO for Release CI builds. They'll probably take longer, but it'll make nightlies more playable. Maybe just for on-merge master-branch builds rather than MRs.

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