QPID-5646: Detect and build against earlier versions of Proton.
- This should detect any version of Proton that either uses
  pkg-config or native cmake config files.

git-svn-id: https://svn.apache.org/repos/asf/qpid/branches/0.28@1582379 13f79535-47bb-0310-9956-ffa450edef68
QPID-5646: Fixes Proton detection to work with cmake 2.6

git-svn-id: https://svn.apache.org/repos/asf/qpid/branches/0.28@1582380 13f79535-47bb-0310-9956-ffa450edef68
QPID-5646: Fixes Proton detection to work with non standard pkg-config location

git-svn-id: https://svn.apache.org/repos/asf/qpid/branches/0.28@1582381 13f79535-47bb-0310-9956-ffa450edef68