I’m fine with the idea of requiring gnu make 3.8+ (or whatever the ancient version that macOS xcode ships is) – if there is compelling pain that can be removed from our Makefile behavior and maintenance by doing so. It is easy enough to build gnu make on any platform that lacks it by default and is trivial to do for all of our PEP-11 tier1-3 platforms.
The larger issue is that we should really move away from autoconf and make entirely. But don’t block on that concept. It’s a separate rabbit hole that has so far not seen anyone with the patience to lead us to settlement on what the actual requirements are.