I had the need to install FreeSWAN on an old RedHat Linux 7.3 machine. I initially tried to do a simple "rpmbuild --rebuild..." but that did not do the trick.
A Nokia CheckPoint Firewall was not receiving OSPF adjacency from a Cisco IOS 12.3 3640 Router. Apparently Cisco released a new feature in 12.3 (and 12.2T) that is ON by default... even though it is NEW. The feature is called, Link-Local Signalling (LLS). LLS confuses OSPF on the Nokia (even though it is RFC compliant) and is consequently rejected.