Add libev 4.20 to the Impala build. This is a dependency of KRPC.
FindLibEv.cmake was taken from Apache Kudu.
Change-Id: Iaf0646533592e6a8cd929a8cb015b83a7ea3008f
Reviewed-on: http://gerrit.cloudera.org:8080/5659
Tested-by: Impala Public Jenkins
Reviewed-by: Henry Robinson <henry@cloudera.com>
