don't add qbs-related flags to CONFIG over and over again

Change-Id: I0ca49213d45ab7ab193266bbbc7f201cecdc6eac
Reviewed-by: Christian Kandeler <christian.kandeler@digia.com>

qtcreator.pro

@@ -65,8 +65,10 @@ minQtVersion(5, 0, 0):exists(src/shared/qbs/qbs.pro) {
     cache(QBS_RELATIVE_PLUGINS_PATH)
     QBS_RELATIVE_SEARCH_PATH = $$relative_path($$QBS_RESOURCES_BUILD_DIR, $$QBS_APPS_DESTDIR)
     cache(QBS_RELATIVE_SEARCH_PATH)
-    QBS_CONFIG_ADDITION = qbs_no_dev_install qbs_enable_project_file_updates
-    cache(CONFIG, add, QBS_CONFIG_ADDITION)
+    !qbs_no_dev_install {
+        QBS_CONFIG_ADDITION = qbs_no_dev_install qbs_enable_project_file_updates
+        cache(CONFIG, add, QBS_CONFIG_ADDITION)
+    }
 }
 
 contains(QT_ARCH, i386): ARCHITECTURE = x86