bug#12450: Remove configure's --without-sync-input option.

[Eli Zaretskii <eliz@gnu.org>]

> From: Stefan Monnier <monnier@iro.umontreal.ca>
> Cc: eggert@cs.ucla.edu,  lekktu@gmail.com,  12450@debbugs.gnu.org
> Date: Sun, 16 Sep 2012 10:58:41 -0400
> 
> > Could you please describe how the SYNC_INPUT code is supposed to work
> > in the following situations:
> >   . keyboard input on a TTY that comes while Lisp is running
> >   . an X event that exposes an Emacs frame in a GUI session, coming
> >     while Lisp is running
> 
> - The keyboard input or the X event causes a signal to be delivered.

Which signals are those, specifically?

> - QUIT processes the pending signals which will read the (keyboard/X11)
>   input and turn it in an event in the event queue (so input-pending-p
>   will know something has arrived), it may also do some more processing
>   such as redraw the window if the GUI needs it.  Currently it also
>   handles things like mouse-face highlighting.

Perhaps you could update the relevant portions in the commentary at
the beginning of xdisp.c, as they tell a somewhat different story
(probably about a soon-to-become-extinct non-SYNC_INPUT mode).

> So SYNC_INPUT basically delays the processing of incoming signals from
> "run it at any time" to "run it at the next safe point" where a safe
> point is defined as "a point where we call QUIT".  "QUIT" can also be
> understood as a sibling of "yield".
> 
> Does that answer your question?

It's a beginning, thanks.