QGtk3Theme: Respect xcb_xlib config option

Change-Id: I4b01a694e8a13a6f009296d8ccfa8f8eb21043e4
Reviewed-by: Liang Qi <liang.qi@qt.io>
(cherry picked from commit dbe858bf80f554c4d916ee230fc9dcde01699bd7)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

src/plugins/platformthemes/gtk3/qgtk3theme.cpp

@@ -11,7 +11,9 @@
 #undef signals
 #include <gtk/gtk.h>
 
+#if QT_CONFIG(xcb_xlib)
 #include <X11/Xlib.h>
+#endif
 
 QT_BEGIN_NAMESPACE
 
@@ -60,13 +62,17 @@ QGtk3Theme::QGtk3Theme()
     else if (QGuiApplication::platformName() == "xcb"_L1)
         gdk_set_allowed_backends("x11,wayland");
 
+#if QT_CONFIG(xcb_xlib)
     // gtk_init will reset the Xlib error handler, and that causes
     // Qt applications to quit on X errors. Therefore, we need to manually restore it.
     int (*oldErrorHandler)(Display *, XErrorEvent *) = XSetErrorHandler(nullptr);
+#endif
 
     gtk_init(nullptr, nullptr);
 
+#if QT_CONFIG(xcb_xlib)
     XSetErrorHandler(oldErrorHandler);
+#endif
 
     /* Initialize some types here so that Gtk+ does not crash when reading
      * the treemodel for GtkFontChooser.

src/plugins/platformthemes/gtk3/qgtk3theme.h

@@ -4,6 +4,7 @@
 #ifndef QGTK3THEME_H
 #define QGTK3THEME_H
 
+#include <private/qtguiglobal_p.h>
 #include <private/qgenericunixthemes_p.h>
 
 QT_BEGIN_NAMESPACE