External Header example

.gitignore

main

conf.h

#ifndef SAMPLE_CONF_H
#define SAMPLE_CONF_H
#define FIRST_MACRO 1
#endif //SAMPLE_CONF_H

hello_city.h

#ifndef SAMPLE_HELLO_CITY_H
#define SAMPLE_HELLO_CITY_H

#define SECOND_MACRO 1

#endif //SAMPLE_HELLO_CITY_H

hello_world.c

#include "hello_city.h"
#include "hello_world.h"
#include <stdio.h>

#if FIRST_MACRO
#if SECOND_MACRO
void  hello_world(void) {
    printf("Hello, World!\n");
}
#endif //SECOND_MACRO
#endif //FIRST_MARCO

#ifndef FIRST_MACRO
void  hello_world(void) {
    printf("Shame on you!\n");
}

#endif

hello_world.h

//
// Created by username on 5/26/18.
//

#ifndef SAMPLE_HELLO_WORLD_H
#define SAMPLE_HELLO_WORLD_H

/* PROJECT_CONF_H might be defined in the project Makefile */
#ifdef PROJECT_CONF_H
#include PROJECT_CONF_H
#endif /* PROJECT_CONF_H */

void hello_world();

#endif //SAMPLE_HELLO_WORLD_H

main.c

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

int main() {
    hello_world();
    return 0;
}

makefile

main: main.o hello_world.o
	gcc -o main main.o hello_world.o
	rm *.o

main.o: main.c hello_world.h
	gcc -c main.c

hello_world.o: hello_world.c hello_world.h hello_city.h
	gcc -DPROJECT_CONF_H=\"conf.h\" -c hello_world.c

clean:
	rm main

all: main