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

[Stefan Monnier]

>> If someone can describe in detail what SYNC_INPUT means

Maybe not quite, because it was implemented rather than designed.
But SYNC_INPUT's purpose is to reduce the code run during POSIX's signal
handling to its minimum (since it's basically unsafe to do anything in
there, due to POSIX's definition of signal handling), so it only sets
a global var (pending_signals) which is then polled by QUIT, so that the
real code reacting to the arrival of a signal is run synchronously.

This avoids problems such as malloc being called in the middle of
another malloc invocation.

The downside of SYNC_INPUT is that we may fail to poll pending_signals
often enough and hence become unresponsive.  Since SYNC_INPUT has been
the default for a long while now, I think we can drop the non-SYNC_INPUT
case, indeed, as suggested by Paul.  But I don't know how it interacts
with the non-X11 code.


        Stefan