Правка исходников

GtkWidget * cancel_button = gtk_button_new_with_mnemonic(_("  Отменить"));

    /* Create the Cancel button. */
    GtkWidget * cancel_button = gtk_button_new_from_stock(GTK_STOCK_CANCEL);
    gtk_button_set_alignment(GTK_BUTTON(cancel_button), 0.0, 0.5);
    g_signal_connect(G_OBJECT(cancel_button), "clicked", G_CALLBACK(cancel_clicked), NULL);
    GtkAccelGroup* accel_group = gtk_accel_group_new();
    gtk_window_add_accel_group(GTK_WINDOW(window), accel_group);
#if GTK_CHECK_VERSION(3,0,0)
    gtk_widget_add_accelerator(cancel_button, "activate", accel_group,
        GDK_KEY_Escape, (GdkModifierType)0, GTK_ACCEL_VISIBLE);
#else
    gtk_widget_add_accelerator(cancel_button, "activate", accel_group,
        GDK_Escape, (GdkModifierType)NULL, GTK_ACCEL_VISIBLE);
#endif
    gtk_box_pack_start(GTK_BOX(controls), cancel_button, FALSE, FALSE, 4);