/// Conditionally print a formatted message to stdout.
///
/// Evaluates `$cond` at runtime; if `true`, calls [`println!`] with the
/// remaining arguments unchanged. When `$cond` is a compile-time constant
/// `false`, the body is still compiled but the optimizer will eliminate it.
///
/// # Example
///
/// ```rust
/// const VERBOSE: bool = true;
/// print_if!(VERBOSE, "step {} loss = {:.4}", epoch, loss);
/// ```