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

[Paul Eggert]

On 09/15/2012 03:18 PM, Richard Stallman wrote:
> I have a vague memory that the sync input code had an important flaw
> -- that for certain display-updating tasks (perhaps
> mouse-highlighting) it could cause a delay until Emacs was read to
> accept input, which the other code could do instantly.
> 
> I don't know if this is true now, and I'm not sure of the memory at all.
> But it would be good to investigate this question.

Yes, that topic came up in 2008 when SYNC_INPUT was made the
default on non-Windows hosts.  That discussion concluded that it's
not a problem in practice, because SYNC_INPUT does not postpone
X11 event processing indefinitely; it postpones it only until the
next QUIT or UNBLOCK_INPUT.  Please see Stefan's comment in
<http://lists.gnu.org/archive/html/emacs-devel/2008-03/msg01368.html>,
along with the surrounding thread.  The remaining issues noted in
that thread all seem to have been fixed since then, for the
SYNC_INPUT case.