bash$ cat add.c
#include <stdio.h>

int main()
{
        int a = 10;
        int b = 20;

#ifdef DEBUG
        printf("a = %d; b = %d\n", a, b);
#endif

        printf("a + b = %d\n", a + b + EXTRA);

        return 0;
}

bash$ cc -DDEBUG -DEXTRA="10" add.c && ./a.out
a = 10; b = 20
a + b = 40