Remove -fexceptions configure test

[Joseph Myers]

On Thu, 15 Oct 2015, Roland McGrath wrote:

> What is the reason for -fexceptions in all those other places?  If
> each is actually a uses-callbacks case, then we should use that
> variable to clarify that in the documentation.  I think that's the
> actual case for all the stdio cases, because they can lead to
> callbacks via _IO_JUMPS.
> 
> Do we have any -fexceptions cases that are not actually uses-callbacks?

I think:

* If a function may be cancelled, it needs -fexceptions.  This includes 
most functions doing I/O unless explicitly using non-cancellable I/O 
functions.

* If the cancellation may occur from within an inline asm, not just at 
function call points, -fasynchronous-unwind-tables is needed as well.  
(I'm not sure what's going on with the few files using 
-fasynchronous-unwind-tables without -fexceptions.)

* If a function uses callbacks, it needs -fexceptions (both because the 
callbacks might use interfaces that can be cancelled, and also because 
they might throw exceptions).

* For all of the above, __THROW must not be used.

More than one of the above may apply to a function.  I don't know what 
makes sense regarding makefile variables in such cases (I/O and callbacks, 
for example).  And I don't know if we actually intend to allow exceptions 
to be thrown and cancellation to occur for arbitrary callbacks.  
(snprintf is marked __THROWNL - but custom printf handlers can be 
registered.)

-- 
Joseph S. Myers
joseph@codesourcery.com