You don't have to use all the legacy stuff. See: https://autotools.io/whosafraid.html. You choose what tests you want to run. No need to check if you have unistd.h if you don't care about that.

As a rewrite using the same user interface but a completely different developer interface, there is mklove (https://github.com/edenhill/mklove). However, this just covers autoconf: you don't get the flexibility of automake. Also, I don't know how complete it is.

A rewrite just to speed up things a little is a bit useless. Autotools are not that slow. And if a rewrite was done, it would be a shame to keep the horrible syntax. What needs to be kept is