Try harder to get matching versions of python libraries and interpreter

This also adds a check whether we succeed. It fixes issue #47 on my system.

There is however still an opportunity for failure: The PythonLibs version becomes the minimal required version for PythonInterp. If PythonLibs chooses a small version of the libraries, PythonInterp could decide that a larger version is just as good. That doesn't happen on my system with both 2.7 and 3.4 installed. I also found no good way to fix it. At least cmake can detect this case now.