Add v8 version detection to configure.ac

Detect if v8 version > 3.19 and define macro V8_POST_3_19_API

Change-Id: Id97e2516e0d06706b3cf5649dbd5743923eb9de3
Reviewed-on: http://review.couchbase.org/30003
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: Filipe David Borba Manana <fdmanana@gmail.com>
Tested-by: Filipe David Borba Manana <fdmanana@gmail.com>