netif.h present but unusable on Mac

The host is an Intel Core 2 Duo processor running Mac OS X version  
10.5.7

I downloaded the JikesRVM version 3.1.0 and having checked that
ia32-osx was supported set out to build it.  The manual says that
it needs GNU Classpath, and that I should download and install
that by doing ant -f build/components/classpath.xml, which I did.

      [exec] checking ifaddrs.h usability... yes
      [exec] checking ifaddrs.h presence... yes
      [exec] checking for ifaddrs.h... yes
      [exec] checking netinet/in_systm.h usability... yes
      [exec] checking netinet/in_systm.h presence... yes
      [exec] checking for netinet/in_systm.h... yes
      [exec] checking netinet/ip.h usability... yes
      [exec] checking netinet/ip.h presence... yes
      [exec] checking for netinet/ip.h... yes
      [exec] checking net/if.h usability... no
      [exec] checking net/if.h presence... yes
      [exec] configure: WARNING: net/if.h: present but cannot be  
compiled
      [exec] configure: WARNING: net/if.h:     check for missing  
prerequisite headers?
      [exec] configure: WARNING: net/if.h: see the Autoconf  
documentation
      [exec] configure: WARNING: net/if.h:     section "Present But  
Cannot Be Compiled"
      [exec] configure: WARNING: net/if.h: proceeding with the  
preprocessor's result
      [exec] configure: WARNING: net/if.h: in the future, the compiler  
will take precedence
      [exec] configure: WARNING:     ##  
-------------------------------- ##
      [exec] configure: WARNING:     ## Report this to  
[email protected] ##
      [exec] configure: WARNING:     ##  
-------------------------------- ##
      [exec] checking for net/if.h... yes

There was a further problem later down where the Ant script said
      [exec] configure: error: The Java compiler /home/cshome/o/ok/ 
Desktop/jikesrvm-3.1.0/components/ecj/3.2/ecj-3.2/ecj failed (see  
config.log, check the CLASSPATH?)
      [exec] checking if /home/cshome/o/ok/Desktop/jikesrvm-3.1.0/ 
components/ecj/3.2/ecj-3.2/ecj works...


Since ...components/ecj doesn't exist, it's not surprising that it
doesn't "work".  This leads me to suspect a problem with the Ant
script.