What steps will reproduce the problem?
1. On mac os x - ./configure CFLAGS='-arch x86_64' LDFLAGS='-arch x86_64'
2. make


What is the expected output? What do you see instead?

/bin/sh ../libtool --mode=compile gcc -DHAVE_CONFIG_H -I. -I. -I../include  
-I../include   -arch x86_64 -Wall -c -o fw-ipfw.lo `test -f 'fw-ipfw.c' || echo 
'./'`fw-ipfw.c
 gcc -DHAVE_CONFIG_H -I. -I. -I../include -I../include -arch x86_64 -Wall -c fw-ipfw.c  -fno-common -DPIC -o .libs/fw-ipfw.o
fw-ipfw.c:43: warning: 'struct ip_fw' declared inside parameter list
fw-ipfw.c:43: warning: its scope is only this definition or declaration, which 
is probably not what you want
fw-ipfw.c: In function 'fr_to_ipfw':
fw-ipfw.c:47: error: dereferencing pointer to incomplete type
fw-ipfw.c:52: error: dereferencing pointer to incomplete type
fw-ipfw.c:53: error: dereferencing pointer to incomplete type
fw-ipfw.c:54: error: dereferencing pointer to incomplete type
fw-ipfw.c:54: error: 'IP_FW_F_IIFNAME' undeclared (first use in this function)
fw-ipfw.c:54: error: (Each undeclared identifier is reported only once
.....
What version of the product are you using? On what operating system?
libdnet-1.12 on mac os x 10.5.5 with xcode-3.1.4