QPID-4724: Fixes Perl bindings for Cmake < 2.8.

To fix how Cmake prior to 2.8 fails to find the Perl libraries, this
change finds and sets the variables correctly.

git-svn-id: https://svn.apache.org/repos/asf/qpid/trunk@1475945 13f79535-47bb-0310-9956-ffa450edef68