bug#34448: --no-build-details means system-name is nil

[Eli Zaretskii <eliz@gnu.org>]

> From: Lars Ingebrigtsen <larsi@gnus.org>
> Date: Thu, 10 Oct 2019 00:26:53 +0200
> Cc: 34448@debbugs.gnu.org
> 
> void
> init_system_name (void)
> {
>   if (!build_details)
>     {
>       /* Set system-name to nil so that the build is deterministic.  */
>       Vsystem_name = Qnil;
>       return;
>     }
> 
> which is called from
> 
> static void
> init_and_cache_system_name (void)
> {
>   init_system_name ();
>   cached_system_name = Vsystem_name;
> }
> 
> which is called from
> 
> DEFUN ("system-name", Fsystem_name, Ssystem_name, 0, 0, 0,
>        doc: /* Return the host name of the machine you are running on, as a string.  */)
>   (void)
> {
>   if (EQ (Vsystem_name, cached_system_name))
>     init_and_cache_system_name ();
>   return Vsystem_name;
> 
> Is --no-build-details just meant to be used during builds?  If so, we
> can just document that, I guess?

Yes, documenting that would be a good compromise, I think.