packaging: spec: add support for EL7 builds

Change-Id: I5faef659a862f720fd02ef219d6a178e099ab66d
Bug-Url: https://bugzilla.redhat.com/1159293
Signed-off-by: Sandro Bonazzola <sbonazzo@redhat.com>
(cherry picked from commit 4b1608d13ccd07fcc0c488d0b36ffe915e94d59f)