QmlDesigner: Skip QmlDesigner for Qt < 5.3

Otherwise it will not build because it requires QQuickWidget.

Change-Id: Idbe40355af3ca3507268e7d002b04e76d90c45b1
Reviewed-by: Eike Ziller <eike.ziller@digia.com>

README

@@ -12,7 +12,9 @@ Mac OS X 10.7 or later
 
 Building the sources requires Qt 5.2.0 or later.
 It is also possible to build with Qt 4.8.x, but in that case the Welcome mode,
-Qt Quick Designer, QML Profiler, and the WinRT and Qbs support are disabled.
+QML Profiler, and the WinRT and Qbs support are disabled.
+The Qt Quick Designer can only be compiled with Qt 5.3.1 or later , because
+it requires QQuickWidget.
 
 Compiling Qt Creator
 ====================

src/plugins/plugins.pro

@@ -76,13 +76,18 @@ isEmpty(IDE_PACKAGE_MODE) {
         updateinfo
 }
 
+minQtVersion(5, 3, 1) {
+    SUBDIRS += qmldesigner
+} else {
+     warning("QmlDesigner plugin has been disabled.")
+     warning("This plugin requires Qt 5.3.1 or newer.")
+}
+
 minQtVersion(5, 2, 0) {
     SUBDIRS += \
-        qmldesigner \
         qmlprofiler \
         welcome
 } else {
-     warning("QmlDesigner plugin has been disabled.")
      warning("QmlProfiler plugin has been disabled.")
      warning("Welcome plugin has been disabled.")
      warning("These plugins need at least Qt 5.2.")