./config wants me to complain:

[Mike]

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

Machine: G4Mac, OS X 10.3.1 Panther (powerpc-apple-darwin7.0.0)

Apple's version of net/if.h is attached.  
/usr/include/sys/appleapiopts.h exists.  Let me know if you need any 
more information about my setup.

Mike

if.h
Description: Text document