NO-JIRA: Work-around bug in older cmake, finds static python library instead of shared.

FindPythonLibs on older versions of cmake (observed on cmake-2.6.4-5.el5.4)
sometimes finds the static archive library (.a) before the shared object
library (.so) This is almost never what you want. Newer
versions (e.g. cmake-2.8.12.2-2.fc20.x86_64) explicitly look for the .so first
and look for the .a library second.

This workaround fixes the problem as follows:
- It is a no-op except on Unix platforms that use the .so prefix.
- On those platforms it does a search for .so only first.
- Finally we do a default search for other platforms (or if .so was not found)

git-svn-id: https://svn.apache.org/repos/asf/qpid/trunk@1620424 13f79535-47bb-0310-9956-ffa450edef68