Look for libcurl_imp as well

Change-Id: I952d06bce9bc31f369be645c8588f9d3b95d9dbc
Reviewed-on: http://review.couchbase.org/29293
Reviewed-by: Trond Norbye <trond.norbye@gmail.com>
Tested-by: Trond Norbye <trond.norbye@gmail.com>