Use CMAKE_INSTALL_PREFIX instead of INSTALL_ROOT

Change-Id: Ib59d16aedeea4e87e410b5379261d603297067f3
Reviewed-on: http://review.couchbase.org/31663
Reviewed-by: Trond Norbye <trond.norbye@gmail.com>
Tested-by: Trond Norbye <trond.norbye@gmail.com>