Add v8 version detection to configure.ac

Detect if v8 version > 3.19 and define macro `V8_POST_3_19_API`

Change-Id: I8af6c5f9b83f977f414397833f320240f0eeca33
Reviewed-on: http://review.couchbase.org/30028
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: Filipe David Borba Manana <fdmanana@gmail.com>
Tested-by: Filipe David Borba Manana <fdmanana@gmail.com>
Reviewed-on: http://review.couchbase.org/37506
Reviewed-by: Volker Mische <volker.mische@gmail.com>
Tested-by: Volker Mische <volker.mische@gmail.com>
Tested-by: Sarath Lakshman <sarath@couchbase.com>