The Windows Phone 8.1 'MessageDialog' API only seems to support two buttons,
despite the documentation for such mentioning support for three. Trying to use
three or more buttons leads to an exception being thrown. As such, any attempt
to use more than two buttons via SDL_ShowMessageBox (on Windows Phone 8.1) will
lead to no message box getting shown, and the call returning an error.
The Win32 MessageBox and dialog APIs are not available in WinRT apps, to note.
More extensive message dialog support might be available at some point, if and
when XAML support is more fully fleshed-out. I'm not certain of this, though.
