The good news is that, since Christian's recent enhancements in Issue32598 to have autoconf use pkg-config to detect OpenSSL header and lib files, configure will now automatically find the MacPorts OpenSSL as long as the MacPorts bin directory (typically /opt/local/bin) is in the shell PATH when you run configure.  MacPorts supplies pkg-config and .pc files for various packages it installs including OpenSSL. (Apple doesn't ship a copy of pkg-config with macOS).  I found this out by accident; thanks, Christian!  The current status quo is also more robust in that MacPorts is not necessarily installed in /opt/local, a problem with PR 5388 as it stands.  I install MacPorts from source to a non-standard location so it would be good for someone to verify that the current 3.7 configure works with a default (/opt/local) binary install of MacPorts.