QmlDesigner: Make it easier to disable QmlDesigner

Exporting DO_NOT_BUILD_QMLDESIGNER disables the build of
the QmlDesigner plugin.

Change-Id: Ia3809268816cd84ef23b872f8df4709e381b3618
Reviewed-by: Eike Ziller <eike.ziller@theqtcompany.com>

src/plugins/plugins.pro

@@ -51,10 +51,16 @@ SUBDIRS   = \
     beautifier \
     qmakeandroidsupport \
     winrt \
-    qmldesigner \
     qmlprofiler \
     welcome
 
+DO_NOT_BUILD_QMLDESIGNER = $$(DO_NOT_BUILD_QMLDESIGNER)
+isEmpty(DO_NOT_BUILD_QMLDESIGNER) {
+    SUBDIRS += qmldesigner
+} else {
+    warning("QmlDesigner plugin has been disabled.")
+}
+
 
 isEmpty(QBS_INSTALL_DIR): QBS_INSTALL_DIR = $$(QBS_INSTALL_DIR)
 exists(../shared/qbs/qbs.pro)|!isEmpty(QBS_INSTALL_DIR): \