Do not build QmlDesigner tests when the plugin is disabled

Change-Id: Iaf7e06be6b81040c36726aa76923b285e2402aef
Reviewed-by: Christian Stenger <christian.stenger@qt.io>

tests/auto/qml/qml.pro

@@ -1,7 +1,6 @@
 TEMPLATE = subdirs
 
 SUBDIRS +=  \
-            qmldesigner \
             qmleditor \
             qmljssimplereader \
             qmlprojectmanager \
@@ -9,3 +8,14 @@ SUBDIRS +=  \
             reformatter \
             qrcparser \
             persistenttrie
+
+DO_NOT_BUILD_QMLDESIGNER = $$(DO_NOT_BUILD_QMLDESIGNER)
+isEmpty(DO_NOT_BUILD_QMLDESIGNER):qtHaveModule(quick-private) {
+    SUBDIRS += qmldesigner
+} else {
+    !qtHaveModule(quick-private) {
+        warning("QmlDesigner plugin has been disabled since the Qt Quick module is not available.")
+    } else {
+        warning("QmlDesigner plugin has been disabled since DO_NOT_BUILD_QMLDESIGNER is set.")
+    }
+}