Auto-detect engine source location for adding private header includes.
Auto-detect engine source location for adding private header includes.

Avoids need for user to modify build files based on their engine location
just because we have to include some private headers to get to the rendering
code we need. We can get to those paths with the build tool.

OSVR/OSVR-Unreal,OSVR/OSVR-Unreal,OSVR/OSVR-Unreal,OSVR/OSVR-Unreal