[CMake] FindPythonLibs should set PYTHON_INCLUDE_DIR instead of PYTHON_INCLUDE_PATH

[Marcel Loose]

Hi all,

In fact the subject line says it all. To be compliant with the naming
conventions proposed in the Modules/readme.txt file, FindPythonLibs
should set PYTHON_INCLUDE_DIR instead of PYTHON_INCLUDE_PATH.

To avoid breaking existing software, it would be best to simply just add
a definition for PYTHON_INCLUDE_DIR. 

Best regards,
Marcel Loose.