Add iperf3.spec and autoconf goop for filling in version number.

Submitted by:	@i2aaron

(cherry picked from commit d1aab0920910a81707e475fe73495ba115278dfd)
Signed-off-by: Bruce A. Mah <bmah@es.net>

Conflicts:
	configure.ac
Merge branch '3.0-STABLE' of github.com:esnet/iperf into 3.0-STABLE