Add support for building libevent

libevent is a mature piece of software and we may stick
with the stable releases there. May be built by using:

cmake -D DEP_VERSION=2.0.21-cb1 ../../build/cbdeps/libevent

Change-Id: I29b9e8773467228932792970c3f09cc187d4bf1b
Reviewed-on: http://review.couchbase.org/44515
Reviewed-by: Dave Rigby <daver@couchbase.com>
Tested-by: Trond Norbye <trond.norbye@gmail.com>