Separate common macros out from jemalloc cmake file

Change-Id: Ia2ec144348f51c197e3d2439943e3cce0f472b0f
Reviewed-on: http://review.couchbase.org/44253
Reviewed-by: Trond Norbye <trond.norbye@gmail.com>
Reviewed-by: Dave Rigby <daver@couchbase.com>
Tested-by: Trond Norbye <trond.norbye@gmail.com>