C51 warning C322: unknown identifier

I am porting some code to 8051 and the existing code uses #define to determine whether or not some functionality is available

i.e. ....

#define HAVE_STRING_H
.
.
.
.

#ifndef HAVE_STRING_H
#include <string.H
#else
#include <homegrown/string.h>
#endif

Any suggestions on how I can suppress or correct C322?

Obviously, I can change

#define HAVE_STRING_H

to

#define HAVE_STRING_H 1 

and then change

#ifdef HAVE_STRING_H

to

#if HAVE_STRING_H 

but I am trying to preserve as much of the original code as I can.

Any help is appreciated and thanks in advance.

Robert