[cmake] only set policy CMP0017 for cmake > 2.8.3

Fix issue #314
Tested with cmake 2.8.1
[cmake] 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 exectuable is in a non-default location.
Also set PYTHON_INCLUDE_DIR2 to advanced.