Cmake fixes

- only set policy CMP0017 for cmake > 2.8.3:

  Fix issue #314
  Tested with cmake 2.8.1
- improve finding of python libs/includes:

 Use the path of the PYTHON_EXECUTABLE as hint for finding libs/includes,
 so it is actually usable when the executable is in a non-default location.
 Also set PYTHON_INCLUDE_DIR2 to advanced.