CBD-1438: Extensions for Sherlock builds.

Support for building Go packages, include with cgo extensions. Build
forestdb, tuqtng, and indexing if they exist.

Change-Id: I91b3f5c4624c7d3fe57923c93c8b50c220017976
Reviewed-on: http://review.couchbase.org/42120
Reviewed-by: Trond Norbye <trond.norbye@gmail.com>
Tested-by: Chris Hillery <ceej@couchbase.com>