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 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.
pkg update pkg upgrade
# pkg install git cmake libmaxminddb miniupnpc pkgconf libnatpmp leveldb websocketpp boost-all python npm-node12
# git clone https://github.com/airdcpp-web/airdcpp-webclient.git
# cd airdcpp-webclient
# cmake .
# make -j2
If everything ran fine, you'll find the airdcppd binary in the airdcppd folder. So:
# make install