I think this is very much debatable, and I believe the FSF at least takes the position that greet.c is a derived work of Y since it is designed to depend on Y. Even more, their position is that a program like this:

  #include <stdlib.h>
  
  int main() {
    create_complex_c_code_using_gcc_specific_extensions("./intermediate.c");
    system("gcc -o intermediate ./intermediate.c");
    system("./intermediate");
  }

Now, this type of thing has never been tested in a court of law as far as I know, so it's difficult to say whose interpretation of copyright and its implications is actually correct. But this is similar to the idea that if you were to write an original novel that uses characters or very recognizable aspects of, say, The Lord of the Rings, you'd be creating a derived work (and thus permission from the Tolkien estate), even if you didn't include any part of the original longer than "Galadriel".