I've seen a couple of people ask about this, so it's probably worth a 
post.  Make sure you install your includes before trying to build 
sysctl(8); it needs just recently added definitions in 
<netinet/tcp_var.h>.

--------------------------------------------------------------------------
Jason R. Thorpe                                       thorpej@nas.nasa.gov
NASA Ames Research Center                               Home: 408.866.1912
NAS: M/S 258-6                                          Work: 415.604.0935
Moffett Field, CA 94035                                Pager: 415.428.6939