C Macro trick to hide assignment?

Question

I have a project with a handful of files: source1.c source2.c source1.h source2.h. Now source1.c declares some variables and source1.h externs them (with a conditional build macro). source2.c would then use the extern variables by assigning a value to them.

The issue is that source1.h has something like the following:

#ifdef CUST1
extern type var_name;
#else
// Need to extern something or use a clever macro here but i dont like these below
//#define var_name int x (requires top of scope block usage)
//#define var_name // (doesnt compile)
//#define var_name -1 (simply doesnt compile)
//#define var_name 1= (This seems like it would reduce to unused 1==1; but doesnt work)
#endif

My build works for CUST1 but it will not work for CUST2 because var_name is never declared/outta scope when it is referenced in source2.c.

I dont want to use var_name for CUST2 and it is unreachable code in source2.c. Here is my question, how can i use a macro to #define var_name so that the assignment "goes away" or does nothing?

I could "#define var_name int x". This would put int x on the stack in the source2.c functions and assign the values in source2.c, but if it is ever referenced anywhere besides the top of a scope block, my old (C89?) compiler will error.

For example if source2.c ever had the following it would fail to compile:

unsigned short local_var = 0;
local_var = 1; //or someother value
var_name = local_var * 2;

I could wrap the logic in source2.c with the same #ifdef CUST1 macros but that seems not so good either.

If var_name was only being compared against it would not be so bad because i could just use #define var_name -1 or something that would fail all compares/switches. The issues is -1 = temp_var; does not compile because -1 cant be an lvalue.

Similiarly i cant "#define var_name //" because comments are removed before the marcos are replaced according to this:Can you #define a comment in C?

Is there a clever macro trick that will hide/remove this assignment, that does not put a local_var on the stack? I feel like there is something possible with a ternary but i cant think of it.

EDIT minimal example code:

source1.c

int var_name = 0;

source1.h

#ifdef CUST1
extern int var_name;
#else
// clever macro
#endif

source2.c

#include "source1.h"
int main(){
  var_name = 1;
  return 0;
}

Answer 1

If the code accessing var_name in source2.c is unreachable when CUST1 is not defined (as you seem to say), then you can define it like this:

#include <stdlib.h>

#define var_name (*(abort(), (type *)0))

This will work syntactically as an lvalue (or rvalue) of the correct type, but will abort the program if it's actually ever executed.