Challenge 1: Building the Three-File Project Manually — Possible Solution
====================================================================

math_utils.h:

#ifndef MATH_UTILS_H
#define MATH_UTILS_H

int add(int a, int b);

#endif

math_utils.c:

#include "math_utils.h"

int add(int a, int b) {
    return a + b;
}

main.c:

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

int main() {
    printf("%d\n", add(2, 3));
    return 0;
}

Build commands:

    $ gcc -c math_utils.c -o math_utils.o
    $ gcc -c main.c -o main.o
    $ gcc math_utils.o main.o -o program
    $ ./program

Output:

    5

WHY THIS WORKS AS AN ANSWER
------------------------------
Each .c file is compiled independently into its own .o file first
(neither compile step needs the other file to exist yet, since the
header alone provides add's declaration), and only the final command
combines both object files into one executable -- exactly the
compile-then-link separation the chapter demonstrates, with the header
guard (Chapter 4) included as good practice even though this specific
example never includes math_utils.h twice.