// add.c
int myadd(int a, int b)
{
    return a+b;
}

// mul.c
int mymul(int a, int b)
{
    return a*b;
}

// div.c
int mydiv(int a, int b)
{
    return a/b;
}

// x.h
int myadd(int a, int b);
int mymul(int a, int b);
int mydiv(int a, int b);

// x.c
#include <stdio.h>
#include "x.h"

int main()
{
    int a = 100;
    int b = 12;
    int add, mul, div;
    add = myadd(a, b);
    mul = mymul(a, b);
    div = mydiv(a, b);
    
    printf("%d + %d = %d\n", a, b, add);
    printf("%d * %d = %d\n", a, b, mul);
    printf("%d / %d = %d\n", a, b, div);
    
    return 0;
}

sudo yum install autoconf automake

sudo apt-get install autoconf

autoscan

mv configure.scan configure.ac

vim configure.ac

aclocal

autoconf

autoheader

vim Makefile.am

Makefile.am文件内容：
AUTOMAKE_OPTIONS=foreign
bin_PROGRAMS=demo
demo_SOURCES= x.c add.c mul.c div.c

automake --add-missing

./configure

make

./demo