The issue seems to be with cmake.

A bunch of dependencies of cmake can be built with cmake. When that happens, they generate cmake information files used by other packages. Some packages outright require these files, they won't build at all without it (I am dealing with one such package right now). So if we say that the dependencies of cmake cannot be built using cmake, then this limits the useful of those packages in other cmake-based builds, or outright prevents the packaging of packages that depend on them.

Perhaps there is a way to manually generate the required information files without actually directly using cmake, or by using a multibuild, but I don't know if that is feasible or reliable.