Improve detection of no Qt usage

Handle .pro-files which remove everything from the QT variable as
not using Qt as well as those .pro files using CONFIG -= qt.

Change-Id: I7cc3740ce256093b7d14593e9ff0a6919fd02615
Reviewed-by: Daniel Teske <daniel.teske@nokia.com>

src/plugins/qt4projectmanager/qt4target.cpp

@@ -275,7 +275,7 @@ QList<ProjectExplorer::ToolChain *> Qt4BaseTarget::possibleToolChains(ProjectExp
     QList<Qt4ProFileNode *> profiles = qt4Project()->allProFiles();
     bool qtUsed = false;
     foreach (Qt4ProFileNode *pro, profiles) {
-        if (!pro->variableValue(QtVar).isEmpty()) {
+        if (pro->variableValue(ConfigVar).contains(QLatin1String("qt")) && !pro->variableValue(QtVar).isEmpty()) {
             qtUsed = true;
             break;
         }