Fix GCC_LINUX_FUTEX to work with C99 compilers

[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.

jeff