Let MSVC treat warnings as errors

Treat warnings printed by MSVC 2012, 2013 as
errors in the modules that declare themselves
warning_clean (for developer builds).

Change-Id: I55fb3bde460e13cfdbd6f59634e3b5335b2dd715
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@theqtcompany.com>

mkspecs/features/qt_common.prf

@@ -79,6 +79,9 @@ warnings_are_errors:warning_clean {
             # Work-around for bug https://code.google.com/p/android/issues/detail?id=58135
             android: QMAKE_CXXFLAGS_WARN_ON += -Wno-error=literal-suffix
         }
+    } else:msvc {
+        # enable for MSVC 2012, MSVC 2013
+        equals(MSVC_VER, "11.0")|equals(MSVC_VER, "12.0"): QMAKE_CXXFLAGS_WARN_ON += -WX
     }
     unset(ver)
 }