I got a build failure on Ubuntu/hoary while doing a full modular build.

make  all-am
make[1]: Entering directory `/home/ds/xorg/app/xdm'
if gcc -DHAVE_CONFIG_H -I. -I. -I.    -I/opt/xorg/include   -I/opt/xorg/include
  -D_XOPEN_SOURCE -D_BSD_SOURCE -DHASXDMAUTH -DSECURE_RPC -g -O2 -MT xdm-auth.o
-MD -MP -MF ".deps/xdm-auth.Tpo" -c -o xdm-auth.o `test -f 'auth.c' || echo
'./'`auth.c; \
then mv -f ".deps/xdm-auth.Tpo" ".deps/xdm-auth.Po"; else rm -f
".deps/xdm-auth.Tpo"; exit 1; fi
gcc  -g -O2   -o xdm  xdm-auth.o xdm-daemon.o xdm-server.o xdm-dpylist.o
xdm-dm.o xdm-error.o xdm-file.o xdm-netaddr.o xdm-reset.o xdm-resource.o
xdm-protodpy.o xdm-policy.o xdm-session.o xdm-socket.o xdm-streams.o xdm-util.o
xdm-xdmcp.o xdm-mitauth.o xdm-genauth.o xdm-access.o xdm-choose.o xdm-greet.o
xdm-Login.o xdm-verify.o xdm-xdmauth.o xdm-prngc.o xdm-rpcauth.o 
-L/opt/xorg/lib -lXaw8 -lXmu -lXt -lSM -lICE -lXpm -lXp -lXext -lX11 -ldl -lXau
  -L/opt/xorg/lib -lXdmcp   -lcrypt -lpam 
xdm-auth.o(.text+0x11aa): In function `DefineSelf':
/home/ds/xorg/app/xdm/auth.c:1074: undefined reference to `IN6_IS_ADDR_LOOPBACK'
collect2: ld returned 1 exit status
make[1]: *** [xdm] Error 1
make[1]: Leaving directory `/home/ds/xorg/app/xdm'
make: *** [all] Error 2