Use CMAKE_INSTALL_PREFIX instead of INSTALL_ROOT

Change-Id: Ie8f34956654ce4de6a7307994637f0398816448b
Reviewed-on: http://review.couchbase.org/31664
Reviewed-by: Trond Norbye <trond.norbye@gmail.com>
Tested-by: Trond Norbye <trond.norbye@gmail.com>