Use FL_OVERRIDE for all overridden virtual methods (#611)

FL_OVERRIDE is defined as `override` for C++11 and higher
FL_OVERRIDE is defined as `override` for VisualC 2015 and newer
Don't interfere with Fl_Widget::override()

test/offscreen.cxx

@@ -48,8 +48,8 @@ public:
     return false;
   }
 private:
-  void draw();
-  int handle(int event);
+  void draw() FL_OVERRIDE;
+  int handle(int event) FL_OVERRIDE;
   // Generate "random" values for the line display
   double random_val(int v) const
   {