My current installation is inside JAIL, inside freenas.
Not going to discuss on setting up jail etc., this topic mainly focused on quick installation once your JAIL is setup.

Installing packages required.

pkg install nginx
pkg install rrdtool
pkg install fcgiwrap

I've built backuppc4, because i needed to choose more options than default.
But you can install using command

cd /usr/ports/sysutils/backuppc4/
make config # Select options you want to install
make -j4
make install

Enable backuppc daemon

sysrc backuppc_enable=YES

Running Nginx going to use fcgiwrap package instead of CGI. Enable wrapper in rc.conf.

/etc/rc.conf

fcgiwrap_enable="YES"
fcgiwrap_profiles="main"
fcgiwrap_socket_owner="www"
fcgiwrap_main_socket="unix:/var/run/fcgiwrap/fcgiwrap.socket"
fcgiwrap_main_user="backuppc"