CMakeLists: Allow building of static libcouchbase

Change-Id: Iada81a450e83daa98c08223bfe1bb258baf8c397
Reviewed-on: http://review.couchbase.org/33333
Reviewed-by: Brett Lawson <brett19@gmail.com>
Tested-by: Mark Nunberg <mnunberg@haskalah.org>