Fix GCC_LINUX_FUTEX to work with C99 compilers

[Florian Weimer]

* Jeff Law:

> On 9/6/19 2:57 AM, Florian Weimer wrote:
>> Without this change, libstdc++ is built without futex symbols if GCC
>> rejects implicit function declarations in default mode.
>> 
>> Thanks,
>> Florian
>> 
>> config/ChangeLog:
>> 
>> 2019-09-06  Florian Weimer  <fweimer@redhat.com>
>> 
>> 	* futex.m4 (GCC_LINUX_FUTEX): Include <unistd.h> for the syscall
>> 	function.
>> 
>> libgomp/ChangeLog, libitm/ChangeLog, libstdc++-v3/ChangeLog:
>> 
>> 2019-09-06  Florian Weimer  <fweimer@redhat.com>
>> 
>> 	* configure: Regenerate.

> Do you think it's worthwhile for me to pause the builder, update the
> compiler with this patch and restart stuff that's failed?  It's not hard
> to do.

Yes, I think it's absolutely necessary.  Anything Qt-related fails to
build right now, and many more C++ programs are affected

Would it help if I backport the patches to the GCC 9 branch?  Or do you
just want to stick it to the existing implicit function declaration
patch?

Thanks,
Florian