[1] Having hacked on GNU make I know this won't be easy (it's quite a mess). In fact, it would probably be easier to replace it entirely and fix some of its other deficiencies in the process.
Plus it will not only optimize GNU Make workloads, but potentially other programs that do that. And to be honest, playing with pipe for signaling (and more rarely token counting) is STILL a must-have way to do IPC portably, esp. if you want to integrate with polling on fds. Various OSes have also various new things, but they are non-portable -- maybe someone should try to get eventfd&friends in POSIX?
Anyway, if that's a "good example of how we develop software these days", I would say we managed to develop software the right way at last, making basic stuff work well before succumbing to the tentation to rewrite everything all the time just to play with new shinny techs, getting fresh bugs in the process.
Also you know what has evolved in the last few years, motivating this perf fix? Availability of HIGHLY parallel computers for not too expensive. Yes, I continue to find it highly refreshing that work is done to get all the boring historical software work better in that context. Maybe you think we can afford rewriting all the things all the time to follow supposedly radical technological changes (and using less well designed OSes?), but if so then this thought is not shared widely or at least not followed with action. Rightly so because economically the outcome would be doubtful: just look at the patch, it is way smaller than an hypothetical shiny new rewrite of GNU Make...
There are already dozens of alternatives to GNU Make. Linux already did something similar with Git.
So, like buggy and hard to understand?