was changing the include to:
#include "wx/osx/core/cfstring.h"
and 'wxMacCFStringHolder' to wxCFStringRef
that seemed to do the trick. I have to admit that I didn't spend much time on investigating it, it seemed to be the first fitting choice. But maybe it helps someone else.
Now I'm stuck at startup with some wxWidgets assert which blocks me entirely since I'm not able to click it away, the window has no focus. Not sure when I will have the time to find out more.
assert 'item' failed in Enable(): attempt to enable an item which doesn't exist.