CMake: add version to cpp apps
CMake: generate bindings only if possible

* Generate python binding if PYTHONLIBS_FOUND.
* Generate java binding if JAVA_FOUND AND JNI_FOUND.
* So theses packages are not required to build AvTranscoder.
CMake: check python version when install corresponding binding

* If CMake 2.8.8, can get python version from PYTHONLIBS_VERSION_STRING.
* Else, get python version from include path.