C macro __CLASSIC_C__

The C macro __CLASSIC_C__ is defined when the compiler is in pre-ANSI (K&R) mode. This is a rather uncommon scenario. It might be used in retrocomputing with very old and unmaintained compilers such as the 1990s Bruce Evans’ C compiler (bcc).

#include <stdio.h>

int main() {

#if defined(__CLASSIC_C__)
  printf("K&R C\n");
#elif defined(__STDC__)
  printf("ANSI C\n");
#else
  printf("Unknown C\n");
#endif

return 0;
}

Reference: Compiler Macros wiki