Or make one you don't understand the purpose of, rewriting GNU Autoconf in GNU make would serve no purpose since what Autoconf does has nothing to do with building, but rather finding out details about the running system.
Autotools in general, I guess. Detecting OS and features, conditionally including sources in the build, abstracting compiler flags. The most important feature though was automatically figuring out the list of source files in an arbitrary file system tree structure and then mapping them to the build tree as object and dependency files. I somehow conjured up a recursive wildcard function that implements a simple find command in pure GNU make. It's kind of horrifying now that I think about it.
I suppose I was rewriting automake too: I started metaprogramming GNU make by evaluating templates. It's a surprisingly lisplike turing tarpit.
Maybe you were thinking of GNU Automake ...