hello.c

#include <stdio.h>
#include "hello2.c"
void print_hello(){
    printf("Hello, World!\n");
}
int main(){
    print_hello();
    print_pi();
    return 0;
}

hello2.c

#include <stdio.h>
#define PI 3.14159

void print_pi(){
    printf("PI ~= %f\n", PI);
}