One of the core dependencies of #syslog_ng is the #ivykis library. We have a few #FreeBSD specific fixes in our ivykis fork, however the syslog-ng port uses ivykis from ports instead of the bundled version. I am now working on making it selectable. #Testing and help are welcome.
https://www.syslog-ng.com/community/b/blog/posts/building-syslog-ng-with-bundled-ivykis-on-freebsd