debian: Avoid unit test failure when doing "unofficial" builds.

debian: Avoid unit test failure when doing "unofficial" builds.

The configure option --with-build-number=0 is interpreted differently in
different places.  The configure script itself accepts 0 as an actual
build number and puts '#define BUILDNR "+build0"' into config.h.  The
code in python/automake.mk treats 0 as "no build number" and puts
'BUILDNR = ""' into version.py.

This commit avoids the problem by not passing 0 as a build number.

Reviewed-by: Simon Horman <horms@verge.net.au>
Signed-off-by: Ben Pfaff <blp@nicira.com>