Disable refcounting type propagation with C++

The type propagation breaks the GRefPtr.h class in WebKitGTK, and in
any case existing C++ code calling the C API will need to perform an
explicit cast, as there's no automatic promotion of pointer types to
and from void*.

Tested-by: GNOME Continuous

https://bugzilla.gnome.org/show_bug.cgi?id=790697