#include <stdio.h>
// #define DEBUG 1

int main() {
#ifdef DEBUG
  printf("All good, DEBUG is initialized");
#else
#error Debug is not initiated!
#endif
  return 0;
}

// #error identifier blocks code compilation if compiler reached it.
// For example code above will not compile if DEBUG is not initiated.
//
// error.c: In function ‘main’:
// error.c:8:2: error: #error Debug is not initiated!
//     8 | #error Debug is not initiated!
//       |  ^~~~~
// make: *** [Makefile:14: main] Error 1