Cmake: fix install python binding file

* Can't use file(GLOB ...) function to get a file which doesn't exist
yet.
* Now no need to reconfigure CMake file after a first build to install
the python binding file avtranscoder.py.
CMake: fix python version

* Never get python version from DEFINED PYTHONLIBS_VERSION_STRING, even
if CMake 2.8.8.
* Get python version from include path.
* This will fix the python stuff when create packages.
* Use PYTHON_INCLUDE_DIR instead of PYTHON_INCLUDE_DIRS (not defined in all CMake versions...).
Merge pull request #91 from cchampet/fix_cmake_python

CMake: fix python binding