fix: install to DATAROOTDIR instead of hardcoding share

[henryiii]

Closes#2571; this is defined to share, and unlike DATADIR, is not sometimes overwritten incorrectly for storing CMake files.

[henryiii]

I don't see any reason this can't go into 2.6.0, as it's just defined to share. This is useful for cross-compiling, as it can be set manually by toolchains. It also should correctly support installing to / if someone wanted to do that, as all GNU paths get usr added.