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()
All standard dialogs like fl_message() now handle command-c (macOS)
and ctrl-c (other platforms) to copy the message text of standard
dialogs to the clipboard as requested by GitHub Issue #388.