[need xsrc to build] I am guessing that the tool build is trying to build some X tools, even if later they will not be used. If this is right, I see two options for you
1) conditionalize the tools build to not build the x tools if MKX11 is off. 2) checkout xsrc as well and decline to be bothered by its presence note that you can build for multiple targets from sources on one big machine, so this doesn't mean every machine has a copy of xsrc. (Also, you say 'headless', but not having a display is separate from not wanting client-side X11, so that one can display remotely. But if you don't want X11 you don't want it, and I can't say you are wrong.)