build.c

void rebuild_if_modified(int argc, char *argv[])
{
    assert(argc > 0);
    const char *binary = argv[0];
    const char *source = __FILE__;

    struct stat source_stat;
    stat(source, &source_stat);

    struct stat binary_stat;
    stat(binary, &binary_stat);

    if (source_stat.st_mtime > binary_stat.st_mtime) {
        printf("%s is modified. Rebuilding myself...\n", source);
        pid_t pid = fork();
        if (pid) {
            int status = 0;
            waitpid(pid, &status, 0);
            execlp(binary, binary, NULL);
        } else {
            printf("Building build\n");
            execlp("gcc", "gcc", "-o", binary, source, NULL);
        }
    }
}