fix building of FLTK widget opcodes with VS
fix building of FLTK widget opcodes with VS

csound/csound,ketchupok/csound,csound/csound,csound/csound,ketchupok/csound,ketchupok/csound,ketchupok/csound,csound/csound,ketchupok/csound,csound/csound,ketchupok/csound,ketchupok/csound,csound/csound,ketchupok/csound,ketchupok/csound,csound/csound,csound/csound,csound/csound,ketchupok/csound,csound/csound