gistfile1.txt

xyz.h:
#ifdef XYZ_HEADER

#ifndef XYZ_HEADER_H
#define XYZ_HEADER_H

void xyz_foo();

#endif
#else

#include <stdio.h>

void xyz_foo(){
    printf("hello from xyz\n");
}

#endif

x.c:
#define XYZ_HEADER
#include "xyz.h"

int main(){
    xyz_foo();
}

$ gcc -x c x.c xyz.h -o x
$ ./x
hello from xyz