make honoring -DPYTHON_LIBRARY more solid

as cmake --help-module FindPythonLibs states
If you'd like to specify the installation of Python to use, you should modify the following cache variables:

         PYTHON_LIBRARY             - path to the python library
         PYTHON_INCLUDE_PATH         - path to where Python.h is found

(please note it is PYTHON_INCLUDE_PATH  (deprecated) and not PYTHON_INCLUDE_DIR (new in 2.8) since we require cmake >= 2.6.2)