Fix CMake Warnings

This fixes those annoying CMake Warnings on linking the Python libraries.
Since CMake 2.8.8, FindPythonLibs automatically decides whether to use the Debug or Release library. Thus, the attempt to specify it again triggered the warnings. Let's now set the CMake minimum version to 2.8.8 and do it properly.

For more information on said library decision magick, you can refer here:
https://github.com/Kitware/CMake/commit/91d5a2a386051cb7903044267c5364402bbe190b