Nuke cmake files as part of make clean

Change-Id: Ia395ff087e3b3394fa41a97d1ad73a9a8a4e8fa7
Reviewed-on: http://review.couchbase.org/30402
Reviewed-by: Trond Norbye <trond.norbye@gmail.com>
Tested-by: Trond Norbye <trond.norbye@gmail.com>