do not define variables in headers!

thirdparty/lwip/src/include/lwip/errno.h

@@ -174,8 +174,7 @@ extern "C" {
 #define  EMEDIUMTYPE    124  /* Wrong medium type */
 
 #ifndef errno
-//extern int errno;
-static int errno;
+extern int errno;
 #endif
 
 #else /* LWIP_PROVIDE_ERRNO */