CheckVariableExists.c

#ifdef CHECK_VARIABLE_EXISTS

extern int CHECK_VARIABLE_EXISTS;

#ifdef __CLASSIC_C__
int main(){
  int ac;
  char*av[];
#else
int main(int ac, char*av[]){
#endif
  if(ac > 1000){return *av[0];}
  return CHECK_VARIABLE_EXISTS;
}

#else  /* CHECK_VARIABLE_EXISTS */

#  error "CHECK_VARIABLE_EXISTS has to specify the variable"

#endif /* CHECK_VARIABLE_EXISTS */