So far, we had been lazy and not generated dependencies for 
debugging-only targets, like some/dir/foo.[is]. It actually
cleans up Rules.make to handle them by the same mechanism, though.

Additionally, make use of $(call if_changed) and $(call if_changed_dep)
more consistent, the only difference now is post-processing of
gcc -MD output, which is done for if_changed_dep but not for if_changed.