configure: Make autoconf fail if libtool is not installed.

Otherwise users get an error later like:
    ./configure: line 5093: syntax error near unexpected token
    `disable-shared' ./configure: line 5093: `LT_INIT(disable-shared)'
It's probably friendlier to make configuration fail earlier.

Signed-off-by: Ben Pfaff <blp@nicira.com>
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>
ofproto: Add more thread safety annotations.

These would have found the problem fixed in commit c7be3f559349 (connmgr:
Fix attempt to take mutex recursively when exiting fail-open.).

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

ovsdbmonitor was poorly maintained and not widely used.

Acked-by: Flavio Leitner <fbl@redhat.com>
Signed-off-by: Ben Pfaff <blp@nicira.com>