If I remember correctly, tup is that build system which will spawn a daemon process to build your project faster. That's a tradeoff I'm not ready to accept for a build system...

Never heard about mbld; and to be honest, it doesn't sound sexy at all. There is no doc in the link you posted; and it seems ultra-specific as well (written/parsed in myrddin, to compile myrddin).

As for your problem, I tried thinking about it and must admit that I couldn't think of anything (which definitely shows the limitations of make(1)).