Add a QtVersion::toolChain() method, replacing Qt4Project::toolChain()

Makes the code nicer, we use the ProFileReader to parse the mkspec.

src/plugins/qt4projectmanager/qt4nodes.cpp

@@ -714,7 +714,6 @@ void Qt4ProFileNode::update()
     // update other variables
     QHash<Qt4Variable, QStringList> newVarValues;
 
-    newVarValues[CxxCompilerVar] << reader->value(QLatin1String("QMAKE_CXX"));
     newVarValues[DefinesVar] = reader->values(QLatin1String("DEFINES"));
     newVarValues[IncludePathVar] = includePaths(reader);
     newVarValues[UiDirVar] = uiDirPaths(reader);