I don't think waf could be considered a fork of scons

> When Thomas Nagy decided that SCons's fundamental issues (most notably the poor scalability) were too complex and time-consuming to fix, he started a complete rewrite which he named Waf.

This is why the alternative build systems seem to never catch on. Everyone rolls their own, or forks, or rewrites instead of working together to improve one enough to "disrupt" autotools (though cmake comes close enough...). A sad but common problem in open source development.