FindPythonLibs: Fix for user installed OS X framework

This fixes some FindPythonLibs bugs in the case where two versions of Python are installed on OS X:
- One is the system Python 2.x in /System/Library/Frameworks and /usr/lib
- The other is a user installed Python 3.x framework in /Library/Frameworks

The following bugs are fixed:
1. If the requested Python version is 2.x, FindPythonLibs returns the header path of Python 3.x by mistake.
2. If the requested version is 3, FindPythonLibs does not find the library (it only finds the headers).