CppTools: set default -std=c++11 for Qt4

qmake from Qt4 does not provide C++ standard and
clang can't properly compile Qt4.8.6 and earlier
with c++1z. Behavior in this commit mimics qmake
from Qt5 which also provides c++11 as a default
standard.

Task-number: QTCREATORBUG-16441
Change-Id: I3d29891d6e47f2367f2b3b2bf4be4d86661924e9
Reviewed-by: Nikolai Kosjar <nikolai.kosjar@qt.io>

src/plugins/cpptools/projectpart.h

@@ -80,7 +80,8 @@ public:
     enum QtVersion {
         UnknownQt = -1,
         NoQt,
-        Qt4,
+        Qt4_8_6AndOlder,
+        Qt4Latest,
         Qt5
     };