Differentiate between C and C++ mode and/or make clear that C++ is always required after autowrap

[jpfeuffer]

I just realized that after the introduction of std::shared_ptr for handling objects underlying extension classes, autowrap is unusable for C-only code.
Not sure if that is already documented somewhere.

Maybe having an own ownership management would circumvent this limitation:
https://cython.readthedocs.io/en/latest/src/userguide/extension_types.html#instantiation-from-existing-c-c-pointers

[jpfeuffer]

This is however probably out of scope for us since we use C++ only.