Section External - stop functions from being optimised out?

[Jeremy Cowgar]

Greetings!

How can I stop the compiler from optimising an external function out of
existance? Here is an example:

Section External

  - say_hello ih:POINTER :INTEGER <-
  (
    "Hello, World!\n".print;
    `IUP_CLOSE`:INTEGER
  );

Section Public

  - main <-
  (
    // code
    `IupSetCallback(@control, "ACTION", say_hello)`;
    // code
  );

The external function "say_hello" is never used directly, thus the compiler
is removing it from the compiled code. How can I tell the compiler to
keep say_hello?

Thanks,

Jeremy