Xrandr with Xinerama as a fallback

Linux's HMDDeviceFactory::EnumerateDevices function checks the /tmp/.X11-unix directory for multiple X servers and uses either XScreenCount or Xinerama's XineramaQueryScreens to iterate over the available screens for the presence of an Oculus Rift.

One thing I'm not sure about is how best to pass the display string back to the user.  Maybe passing it in another constructor which takes a String for the display?

Also; a few fixes were made to the CMake files for building on Linux.