USES=cmake: be explicit about incompatibilities with ninja

Document how USES=gmake and USES=emacs are incompatible with
the default USES=cmake behavior of using ninja as generator.
Using a naive USES=cmake emacs line used to get you a strange
ninja error when ${MAKE_ARGS} is passed to it, now there is a
meaningful error message beforehand.

The linked PR examines ways to make it "just work", but that
got bogged down. Do a partial fix so it is easier to track
down next time.

PR: 251191