Make X and/or wayland optional depending on how qtbase was compiled.

--- a/src/VBox/Frontends/VirtualBox/src/platform/nix/VBoxUtils-nix.cpp
+++ b/src/VBox/Frontends/VirtualBox/src/platform/nix/VBoxUtils-nix.cpp
@@ -66,9 +66,12 @@ VBGHDISPLAYSERVERTYPE NativeWindowSubsystem::displayServerType()
 {
     if (!qApp)
         return VBGHDISPLAYSERVERTYPE_NONE;
+#if QT_CONFIG(xcb)
     QNativeInterface::QX11Application *pX11App = qApp->nativeInterface<QNativeInterface::QX11Application>();
     if (pX11App)
         return VBGHDISPLAYSERVERTYPE_X11;
+#endif
+#if QT_CONFIG(wayland)
 #if QT_VERSION >= QT_VERSION_CHECK(6, 5, 0) && defined(RT_OS_LINUX)
     QNativeInterface::QWaylandApplication *pWaylandApp = qApp->nativeInterface<QNativeInterface::QWaylandApplication>();
     if (pWaylandApp)
@@ -76,6 +79,7 @@ VBGHDISPLAYSERVERTYPE NativeWindowSubsystem::displayServerType()
 #else
     if (QGuiApplication::platformName().contains("wayland", Qt::CaseInsensitive))
         return VBGHDISPLAYSERVERTYPE_PURE_WAYLAND;
+#endif
 #endif
     return VBGHDISPLAYSERVERTYPE_NONE;
 }
@@ -100,6 +104,7 @@ bool NativeWindowSubsystem::X11IsCompositingManagerRunning()
 
 bool NativeWindowSubsystem::WaylandIsCompositingManagerRunning()
 {
+#if QT_CONFIG(wayland)
 #if QT_VERSION >= QT_VERSION_CHECK(6, 5, 0) && defined(RT_OS_LINUX)
     QNativeInterface::QWaylandApplication *pWaylandApp = qApp->nativeInterface<QNativeInterface::QWaylandApplication>();
     if (pWaylandApp)
@@ -107,6 +112,7 @@ bool NativeWindowSubsystem::WaylandIsCompositingManagerRunning()
         if (pWaylandApp->compositor())
             return true;
     }
+#endif
 #endif
     return false;
 }
@@ -741,12 +747,14 @@ void NativeWindowSubsystem::setXwaylandMayGrabKeyboardFlag(QWidget *pWidget)
 Display *NativeWindowSubsystem::X11GetDisplay()
 {
     Display *pDisplay = 0;
+#if QT_CONFIG(xcb)
     if (qApp)
     {
         QNativeInterface::QX11Application *pX11App = qApp->nativeInterface<QNativeInterface::QX11Application>();
         if (pX11App)
             pDisplay = pX11App->display();
     }
+#endif
     Assert(pDisplay);
     return pDisplay;
 }
@@ -754,12 +762,14 @@ Display *NativeWindowSubsystem::X11GetDisplay()
 xcb_connection_t *NativeWindowSubsystem::X11GetConnection()
 {
     xcb_connection_t *pConnection = 0;
+#if QT_CONFIG(xcb)
     if (qApp)
     {
         QNativeInterface::QX11Application *pX11App = qApp->nativeInterface<QNativeInterface::QX11Application>();
         if (pX11App)
             pConnection = pX11App->connection();
     }
+#endif
     Assert(pConnection);
     return pConnection;
 }