Problem subdir configures get run again after toolchain built if make repeated.

[DJ Delorie]

Ok, I see it.  It's another "real file depends on phony target" case.
We have essentially this:

	$(module)/multilib.out : maybe-all-gcc
		$(CC_FOR_TARGET) --print-multilibs > $(module)/multilib

We should probably depend on the actual compiler driver, or have some
move-if-changed master copy of multilib.out and have the others depend
on that instead.  Comments?