Make projects buildable from sdist

Main blocker is that there are a lot of `include` that point outside of the project root. We could consider using symlinks instead. Generally windows should support symlinks, but I always hear conflicting reports about it, so I am not sure how trustworthy it is these days. But when creating the sdist, the symlinks are copied over when they are pointing outside of the project root anyway, so in most realistic situations we should be safe to use them?