Makefile: Only set git version if BIRD is build from its repository.

Thanks to Toke Høiland-Jørgensen <toke@toke.dk> for reporting this bug.
32 jobs for int-new in 4 minutes and 43 seconds (queued for 8 seconds)
Status Name Job ID Coverage
  Image
passed docker_centos-6-amd64 #134749
dind

00:00:29

passed docker_centos-7-amd64 #134750
dind

00:00:32

passed docker_debian-7-amd64 #134739
dind

00:00:29

passed docker_debian-7-i386 #134743
dind

00:00:31

passed docker_debian-8-amd64 #134740
dind

00:00:28

passed docker_debian-8-i386 #134744
dind

00:00:32

passed docker_debian-9-amd64 #134741
dind

00:00:32

passed docker_debian-9-i386 #134745
dind

00:00:29

passed docker_debian-testing-amd64 #134742
dind

00:00:31

passed docker_debian-testing-i386 #134746
dind

00:00:31

passed docker_fedora-25-amd64 #134747
dind

00:00:30

passed docker_fedora-26-amd64 #134748
dind

00:00:32

passed docker_opensuse-42_3-amd64 #134751
dind

00:00:30

passed docker_ubuntu-14_04-amd64 #134752
dind

00:00:30

passed docker_ubuntu-16_04-amd64 #134753
dind

00:00:29

 
  Build
passed build-centos-6-amd64 #134760
docker linux amd64

00:00:45

passed build-centos-7-amd64 #134761
docker linux amd64

00:01:06

passed build-debian-7-amd64 #134754
docker linux amd64

00:00:48

passed build-debian-7-i386 #134765
docker linux amd64

00:01:11

passed build-debian-8-amd64 #134755
docker linux amd64

00:01:14

passed build-debian-8-i386 #134766
docker linux amd64

00:00:50

passed build-debian-9-amd64 #134756
docker linux amd64

00:00:55

passed build-debian-9-i386 #134767
docker linux amd64

00:01:20

passed build-debian-testing-amd64 #134757
docker linux amd64

00:01:11

passed build-debian-testing-i386 #134768
docker linux amd64

00:00:50

passed build-fedora-25-amd64 #134758
docker linux amd64

00:00:49

passed build-fedora-26-amd64 #134759
docker linux amd64

00:01:07

passed build-freebsd-11-amd64 #134769
freebsd amd64

00:02:57

passed build-freebsd-11-i386 #134770
freebsd i386

00:03:02

passed build-opensuse-42_3-amd64 #134762
docker linux amd64

00:00:46

passed build-ubuntu-14_04-amd64 #134763
docker linux amd64

00:01:12

passed build-ubuntu-16_04-amd64 #134764
docker linux amd64

00:00:47