QmlDesigner: Check for qmldir of QtQuick.Controls

If QtQuick Controls 1 are missing there is no reason to
build QmlDesigner.

Change-Id: Ic8777fef6addc460b05c8218d96aa13379f02cea
Reviewed-by: Mitch Curtis <mitch.curtis@qt.io>
Reviewed-by: Tim Jenssen <tim.jenssen@qt.io>

src/plugins/plugins.pro

@@ -77,7 +77,11 @@ qtHaveModule(designercomponents_private) {
 
 DO_NOT_BUILD_QMLDESIGNER = $$(DO_NOT_BUILD_QMLDESIGNER)
 isEmpty(DO_NOT_BUILD_QMLDESIGNER):qtHaveModule(quick-private) {
-    SUBDIRS += qmldesigner
+    exists($$[QT_INSTALL_QML]/QtQuick/Controls/qmldir) {
+       SUBDIRS += qmldesigner
+    } else {
+        warning("QmlDesigner plugin has been disabled since Qt Quick Controls 1 are not installed.")
+    }
 } else {
     !qtHaveModule(quick-private) {
         warning("QmlDesigner plugin has been disabled since the Qt Quick module is not available.")