Need to support other windowing systems on Linux
X11 is great, but the windowing systems on Linux are changing. In order for certain distributions to become supported, libraries like Wayland and Mir will need to be used instead. While we're at it, might as well look into using something like XCB instead?
I personally have limited experience with Linux and Linux Windowing Systems. A project of this caliber is probably best suited for somebody more experienced. Also I have no idea if X11 is still compatible, if Mac is any indication, switching libraries dynamically is most likely the best approach.
If you need windows in Linux you could use SDL2 or anything like that. These libraries are very minimal and to the point while still being able to handle all the corner cases of X11 and such. Or you could base your cross platform windowing needs on wxWidgets.
Hey, please see my response here: https://github.com/Robot/robot/issues/10#issuecomment-214964372