Your package fails to build with the newest autoconf-2.71, which was part of a recent wide Fedora change. More information about Fedora autoconf Change available here: https://fedoraproject.org/wiki/Changes/Autoconf_271#How_To_Test. The easiest way to reproduce the problem is to execute a fedora scratch-build on your package. Thank you for cooperation!
stunnel-5.62-1.fc36 builds fine for me against autoconf-2.71. Since autoconf-2.71 is already in rawhide, I think this is outdated or has been fixed somewhere along the way. I am closing this as WORKSFORME. Please re-open if the problem persists.