Where to find OpenSSL devel package - can't seem to find it in Brew
Just install openssl via Homebrew. It contains all you need.

EDIT: I tried building UnrealIRCd with openssl installed and it built successfully.

EDIT2: On a number of package managers, a software package is divided into runtime (used by other softwares to run) and development (used to build the other softwares) packages.
For instance, openssl is a runtime and openssl-devel is a development package on those package managers.
However, packages on Homebrew aren't separated in this manner but just contain both.
This is why just installing openssl is fine on Homebrew.