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

[Stefan Monnier <monnier@iro.umontreal.ca>]

>> > 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?

I'm not familiar enough to be sure (I changed the code of the signal
handler, but not the code that sets it up), but I think it's SIGIO
(basically, both the tty and the X11 code end up having
a file-descriptor on which inputs arrive, and the code sets up this
file-descriptor so that the kernel sends us a signal when input is
ready).

>> - 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).

I'll try and take a look, thanks.


        Stefan