Install a cmakefile for libevent

Change-Id: I42725c9382f894fbff857f16018cbf25e318f9d9
Reviewed-on: http://review.couchbase.org/28734
Reviewed-by: Trond Norbye <trond.norbye@gmail.com>
Tested-by: Trond Norbye <trond.norbye@gmail.com>