Win32: Make waring 4013 error

```
'function' undefined; assuming extern returning int
The compiler encountered a call to an undefined function.
```

Author: nobu

win32/Makefile.sub

@@ -279,7 +279,7 @@ COMPILERFLAG  = -Zm600
 !if $(MSC_VER) >= 1400
 WARNFLAGS = -W2 -wd4100 -wd4127 -wd4210 -wd4214 -wd4255 -wd4574 \
 	    -wd4668 -wd4710 -wd4711 -wd4820 -wd4996 \
-	    -we4028 -we4142 -we4047
+	    -we4028 -we4142 -we4047 -we4013
 !else
 WARNFLAGS = -W2
 !endif