Improve OPENMW_LTO_BUILD CMake option

Currently, the OPENMW_LTO_BUILD CMake option just adds some compiler flags on MSVC. It would be better to make it work universally.

The best way to do so would be to hook it up to https://cmake.org/cmake/help/v3.16/prop_tgt/INTERPROCEDURAL_OPTIMIZATION.html as that should work with all build tools that CMake knows support it. It's less maintenance burden as we don't need to keep track of which flags do what.

Apparently as of CMake 3.9-ish, turning this on when your tools can't do LTO will cause an error, so if we don't want that (e.g. want to change it to a warning) we need to check https://cmake.org/cmake/help/v3.16/module/CheckIPOSupported.html#module:CheckIPOSupported first.

Thankfully, CMake have done pretty much all of this for us, so we can just copy https://gitlab.kitware.com/cmake/cmake/blob/master/CMakeLists.txt#L135-145 and use our option name instead of theirs.