Fix GCC_LINUX_FUTEX to work with C99 compilers

[Jakub Jelinek]

On Fri, Sep 06, 2019 at 10:57:06AM +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.

Ok, thanks.

	Jakub