autuconf error

[Cole]

Hi

I have a freebsd 4.9-Stable box, and i get this problem when running the 
configure script for ntop-3.0.
My autoconf version is:
autoconf-2.57       Automatically configure source code on many Un*x platforms

checking if.h presence... yes
configure: WARNING: if.h: present but cannot be compiled
configure: WARNING: if.h: check for missing prerequisite headers?
configure: WARNING: if.h: proceeding with the preprocessor's result
configure: WARNING:     ## ------------------------------------ ##
configure: WARNING:     ## Report this to address@hidden ##
configure: WARNING:     ## ------------------------------------ ##
checking for if.h... yes

If there is anything you can tell me, it would be much appreciated.
Thanks
/Cole