debug:  Allow MWC 2.3 without __PRETTY_FUNCTION__

This code was brokenly checking for Metrowerks C++ 0.3, not 3.0, but 2.4
supports __PRETTY_FUNCTION__ anyway.  Metrowerks C++ 2.3 doesn't support
either __func__ or __PRETTY_FUNCTION__, so fall back to "(unknown)".