Search and detect jemalloc

See http://www.canonware.com/jemalloc/

Change-Id: I99d8180f163d6018ce7a4a7af49b18163190375f
Reviewed-on: http://review.couchbase.org/37440
Reviewed-by: Dave Rigby <daver@couchbase.com>
Tested-by: Trond Norbye <trond.norbye@gmail.com>
Reviewed-by: Trond Norbye <trond.norbye@gmail.com>