QPID-4776: Fixes building Ruby bindings on Cmake < 2.8

This patch fixes issues regarding finding the Ruby install and building
the bindings for it.

Contributed by Irina Boverman <iboverma@redhat.com>

git-svn-id: https://svn.apache.org/repos/asf/qpid/branches/0.22@1477135 13f79535-47bb-0310-9956-ffa450edef68