Windows MSVC port

Extend the windows port so it compiles with the toolchain from Visual Studio 2013
diff --git a/py/mpconfig.h b/py/mpconfig.h
index 9acfc14..9ff3dd7 100644
--- a/py/mpconfig.h
+++ b/py/mpconfig.h
@@ -312,4 +312,6 @@
 #endif //INT_FMT
 
 // Modifier for function which doesn't return
+#ifndef NORETURN
 #define NORETURN __attribute__((noreturn))
+#endif