The code in configure regarding testing and setting flags for building "headless only" or "headless + headful" is not very clear, partly due to confusion in the old build system.

While theoretically correct, --disable-headful is not the best name in the world. (And yes, I made that up once upon a time :-)) Perhaps --enable-headless-only would be more clear?

Also, we should check what variables we set. Some have strange names, and are treated like bad legacy stuff, even though they are introduced by us and part of build-infra.