Just keeping track of the combination of all operating systems, compilers, crosscompilers, in- and out-of-tree builds makes any change here almost impossible to get right, so adding build options into the mix won't really help. The platforms already use different paths for the SystemDataPath, but the repository layout as well as well as the two cmake build tree layouts are the same on every operating system. I'm fine with "out-of-tree only if you can create symlinks yourself or just want to install or build an installer", as that was the case since we started using CMake, and I would like to keep the complexity of this part of the whole as low as possible, but I'm not opposed to making it better.