IP-Filter header files in FreeBSD

When I configure Squid 2.2S4 in FreeBSD 3.2 with --enable-ipf-transparent,
I get the following message:

WARNING: Cannot find necessary IP-Filter header files
         Transparent Proxy support WILL NOT be enabled

What header files is it looking for? I have nearly the entire source tree
installed in /usr/src, so I think it's looking for something else. I
couldn't find anything in the doc or FAQ, so was wondering if someone
could point in a general direction. TIA

--
"Your illogical approach ... does have its advantages."
				-- Spock, after being Checkmated by Kirk