Fix GCC_LINUX_FUTEX to work with C99 compilers

[Jonathan Wakely]

On 06/09/19 10:57 +0200, 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.

I can't approve the top-level part but it's OK from the libstdc++ side
(for release branches too if desired).