Build: Add support for shared libraries and versioning.

These changes allow for the building of shared libraries by providing
the --enable-shared option to configure. In particular, lib/libopenvwitch.so,
lib/libsflow.so, ofproto/libofproto.so, and ovsdb/libovsdb.so will be built.
Original behavior of building static remains the same.

Additionally, versioning is introduced to each of the libraries objects
paving the way for APIs to be built around them. A detailed comment
outlining the rules for changing a version number is provided in
configure.ac. Note that at this time, the version number is set to
1.0.0, no API is specified yet, and there are no requirements to
maintain any sort of compatibility in any of the libraries.

Signed-off-by: Scott Mann <smann@noironetworks.com>
Signed-off-by: Ben Pfaff <blp@nicira.com>