bug#50043: 28.0.50; USABLE_SIGOI undef code paths do not work correctly

[Eli Zaretskii]

> From: Lars Ingebrigtsen <larsi@gnus.org>
> Date: Fri, 13 Aug 2021 13:56:40 +0200
> 
> 
> To reproduce the problem on Debian (at least), edit src/config.h to
> this:
> 
> /* Define to 1 if SIGIO is usable. */
> /* #define USABLE_SIGIO 0 */
> 
> Then recompile Emacs and say
> 
> (let ((value 'SECONDARY)
>       (x-selection-timeout 2000))
>   ;;; from org-get-x-clipboard in lisp/org-compat.el
>   (gui-get-selection value 'UTF8_STRING)
>   (gui-get-selection value 'COMPOUND_TEXT)
>   (gui-get-selection value 'STRING)
>   (gui-get-selection value 'TEXT))
> 
> This will hang for two seconds instead of returning immediately if
> USABLE_SIGIO is defined.

can you tell where does it hang, and why?  Bonus points for explaining
why the same scenario doesn't hang for USABLE_SIGIO platforms.