windows: check for nonconforming swprintf arguments

Newer versions of mingw do include the correct API for swprintf.
Check for _CRT_NON_CONFORMING_SWPRINTFS as specified in the swprintf
documentation:
http://msdn.microsoft.com/en-us/library/ybk95axf(v=vs.90).aspx

src/win/fs.c

@@ -715,10 +715,10 @@ void fs__readdir(uv_fs_t* req) {
     uv_fatal_error(ERROR_OUTOFMEMORY, "malloc");
   }
 
-#ifdef _MSC_VER
-  swprintf(path2, len + 3, fmt, pathw);
-#else
+#ifdef _CRT_NON_CONFORMING_SWPRINTFS
   swprintf(path2, fmt, pathw);
+#else
+  swprintf(path2, len + 3, fmt, pathw);
 #endif
   dir = FindFirstFileW(path2, &ent);
   free(path2);