libbnpx11: Don't use non-existent macros

The GTK_WIDGET_REALIZED and GDK_WINDOW_XWINDOW macros are gone in GTK+
3.x, so we use their replacement.

Also enable gcc warnings when building libbnpx11, so that we get
warnings for such situations in the future.

Signed-off-by: Bertrand Lorentz <bertrand.lorentz@gmail.com>