Fix endless evaluate loop for invalid qts

Change-Id: Id4ba165f95876cc0b986f5db7e6a788c3fdb2dc0
Reviewed-on: http://codereview.qt-project.org/5709

Reviewed-by: Daniel Teske <daniel.teske@nokia.com>

src/plugins/qt4projectmanager/qt4buildconfiguration.cpp

@@ -378,6 +378,9 @@ void Qt4BuildConfiguration::setToolChain(ProjectExplorer::ToolChain *tc)
     if (tc != 0 && !qt4Target()->possibleToolChains(this).contains(tc))
         return;
 
+    if (toolChain() != tc)
+        return;
+
     BuildConfiguration::setToolChain(tc);
 
     emit proFileEvaluateNeeded(this);