I don't know why Xrandr is required, if it's never used. With this patch mars builds fine.
I'll look into it, thanks for the patch.
It was neded some time ago, maybe it was a dependency of sfml2 and has been dropped.
SFML2 depends on Xrandr, but it doesn't affect mars. As long as you don't call directly any of Xrandr methods, you don't need to link to it.