Update build requirements.

Update build requirements.

Libtool is now required as of commit 38b7a52b61 (openvswitch: Use libtool
and allow building shared libs).

It seems that a build requirement for Python slipped in a while back, for
generating ovs-vswitchd.conf.db.5, and no one complained, so we might as
well make it official.  (That will let us simplify some bits of the build,
too, since they won't have to be conditional on Python anymore, so I'm all
in favor of this change.)

Signed-off-by: Ben Pfaff <blp@nicira.com>