DNF 3: better performance and a move to C++

Figuring out whether the dependency graph changed is one of the things that a good build system will do for you rather than having you think about whether you need to invoke the meta build system or not. And in fact the two-stage build systems do do that, I don't have to re-run the meta build system every time I add an #include in a .c file even though that does change the dependency graph as there is now an edge from the .c to the .h file. I don't see why other changes to the build graph should be handled differently.

As for your other argument, I don't buy that either. Release and debug builds should simply be different targets, and the build system should give you sufficient means of abstraction so that the common parts can be easily shared between the two.