First of all, make sure to refer and understand the general instructions in the official AirDC++ site:
https://airdcpp-web.github.io/docs/installation/compiling.html
What you'll find here are the specific instructions to compile airdcppd in a FreeBSD system, or in a FreeNAS jail.
This was originally tested in a FreeNAS-11.3-U4.1, which uses FreeBSD 11.3-RELEASE-p11 as base system. I don't know if it will work on previous or newer versions, but I intend to keep this document updated.