CMakeList: Add convenience macro to declare utility libraries

Change-Id: Iac689cae1d3e5f0e7a95054da34b283c282de212
Reviewed-on: http://review.couchbase.org/37330
Tested-by: Mark Nunberg <mnunberg@haskalah.org>
Reviewed-by: Brett Lawson <brett19@gmail.com>