test.c:

#include <stdio.h>
#include "test.h"

void __attribute__((weak)) test(void){
  printf("Hi\n");
}

int main(void){
  if(test)
    test();
  return 0;
}

test2.c:

#include "test.h"
#include <stdio.h>

void test(void){
  printf("Hi2\n");
}

test.h:

#ifndef __TEST_H__
#define __TEST_H__

void test(void);

#endif