Autoconf is an extensible package of m4 macros that produce shell
scripts to automatically configure software source code packages.
These scripts can adapt the packages to many kinds of UNIX-like
systems without manual user intervention.  Autoconf creates a
configuration script for a package from a template file that lists the
operating system features that the package can use, in the form of m4
macro calls.

The FSF would make you believe that only gnu-m4 can handle autoconf.
As of OpenBSD 2.7, this is no longer true. This package does not depend
on gnu-m4.