Do correct me if I am doing it wrong now but after reading Robert's mail I uncommented the -Werror... in debian/rules.
When I started building the package again and it seems that quite a few files unde sbin/ifconfig/ is missing netinet/ether.h.
sbin/ifconfig/ifbridge.c
sbin/ifconfig/ifieee80211.c
sbin/ifconfig/af_link.c
Also a few in usr.bin and usr.sbin complained and failed to build, but build with netinet/ether.h.
usr.bin/netstat/if.c
usr.bin/netstat/route.c
usr.sbin/arp/arp.c
Arp is also broken in jessie.
$ arp -a
Segmentation fault
usr.sbin/ppp/tty.c also complains and fails on lines 244.
With these changes though both arp and ifconfig works.
# ./sbin/ifconfig/ifconfig
nfe0: flags=8843<UP,BROADCAST,RUNNING,SIMPLEX,MULTICAST> metric 0 mtu 1500
options=82008<VLAN_MTU,WOL_MAGIC,LINKSTATE>
ether 0:30:5:fa:70:14
inet6 fe80::230:5ff:fefa:7014%nfe0 prefixlen 64 scopeid 0x3
inet XXX.XXX.XXX.X netmask 0xffffff00 broadcast XXX.XXX.X.255
media: Ethernet autoselect (100baseTX <full-duplex>)
status: active
nd6 options=23<PERFORMNUD,ACCEPT_RTADV,AUTO_LINKLOCAL>
pflog0: flags=0<> metric 0 mtu 33152
nd6 options=23<PERFORMNUD,ACCEPT_RTADV,AUTO_LINKLOCAL>
pfsync0: flags=0<> metric 0 mtu 1500
nd6 options=23<PERFORMNUD,ACCEPT_RTADV,AUTO_LINKLOCAL>
lo0: flags=8049<UP,LOOPBACK,RUNNING,MULTICAST> metric 0 mtu 16384
options=600003<RXCSUM,TXCSUM,RXCSUM_IPV6,TXCSUM_IPV6>
inet6 fe80::1%lo0 prefixlen 64 scopeid 0x6
inet 127.0.0.1 netmask 0xff000000
inet6 ::1 prefixlen 64
nd6 options=21<PERFORMNUD,AUTO_LINKLOCAL>
vlan20: flags=8843<UP,BROADCAST,RUNNING,SIMPLEX,MULTICAST> metric 0 mtu 1500
ether 0:30:5:fa:70:14
inet 10.0.0.1 netmask 0xffffff00 broadcast 10.0.0.255
inet6 fe80::230:5ff:fefa:7014%vlan20 prefixlen 64 scopeid 0x7
vlan: 20 parent interface: nfe0
media: Ethernet autoselect (100baseTX <full-duplex>)
status: active
nd6 options=23<PERFORMNUD,ACCEPT_RTADV,AUTO_LINKLOCAL>
# ./usr.sbin/arp/arp -a
? (XXXXX) at 0:1c:7f:33:ba:4a on nfe0 expires in 1111 seconds [ethernet]
? (XXXXX) at 0:19:6:60:23:40 on nfe0 expires in 1197 seconds [ethernet]
? (XXXXX) at 0:1c:7f:33:ba:4a on nfe0 expires in 1034 seconds [ethernet]
gateway (XXXXX) at 0:1c:7f:33:ba:4a on nfe0 expires in 1111 seconds [ethernet]
trashcan (XXXXX) at 0:30:5:fa:70:14 on nfe0 permanent [ethernet]
PS: I apologize for spamming Robert personally.