PROTON-465: FindPerlLibs fails on Ubuntu 12

If the search for libperl.so fails using the standard search in CMake,
then this alternative tool kicks in and searchs other known locations
for the file.

git-svn-id: https://svn.apache.org/repos/asf/qpid/proton/trunk@1628177 13f79535-47bb-0310-9956-ffa450edef68