Well I just tried #define int void in C and C++ before a "hello world" program. C++ catches it because main() has to be an int, but C doesn't care. I think it is because it just treats main() as an int by default; older books on C don't even include the "int" part of "int main()" because it's not strictly necessary.
#define int void replaces all ints with type void, which is typically used to write functions with no return value.