The default configuration calls "format-json". Obviously, this breaks when syslog-ng was compiled without JSON support, preventing the daemon from launching.
At first, it's need to move the strings from /pkg-plist to the /Makefile:
sysutils/syslog-ng is affected too.
The tisket is similar to bug #208441
This port has expired, can you try a supported version, like syslog-ng322?