Okay, this one I've diagnosed. It turns out to be caused by not
defining any BPF filters (which is the case for INSTALL) kernels...

"Perry E. Metzger" writes:
> 
> Just got another unpleasant failure while building a semi-GENERIC
> kernel on port-i386:
> 
> cc  -Werror -Wall -Wmissing-prototypes -Wstrict-prototypes -O2 -I. -I../../..
/../arch -I../../../.. -nostdinc -DI386_CPU -DI486_CPU -DI586_CPU -DMATH_EMULAT
E -DMACHINE_NONCONTIG -DTIMEZONE="0" -DDST="0" -DSWAPPAGER -DVNODEPAGER -DDEVPA
GER -DRAMDISK_SERVER="0" -DRAMDISK_HOOKS -DMINIROOTSIZE="0x200" -DCOMPAT_10 -DC
OMPAT_11 -DCOMPAT_12 -DEXEC_SCRIPT -DEXEC_AOUT -DLKM -DFFS -DMFS -DNFSCLIENT -D
CD9660 -DMSDOSFS -DFDESC -DFIFO -DKERNFS -DINET -DMAXUSERS=32 -D_KERNEL  -Di386
  -c ../../../../net/if_fddisubr.c
> cc1: warnings being treated as errors
> ../../../../net/if_fddisubr.c: In function `fddi_output':
> ../../../../net/if_fddisubr.c:349: warning: label `queue_it' defined but not 
used
> *** Error code 1