bug#32189: 27.0.50; GCC 7 warning due to -Wformat-truncation=2

[Eli Zaretskii <eliz@gnu.org>]

> From: Ken Brown <kbrown@cornell.edu>
> Date: Tue, 17 Jul 2018 15:26:34 -0400
> 
> I'm getting the following warning when building the master branch on 
> Cygwin with GCC 7.3:
> 
> In file included from /usr/include/stdio.h:800:0,
>                   from ../lib/stdio.h:43,
>                   from ../../master/src/w32cygwinx.c:22:
> ../../master/src/w32cygwinx.c: In function ‘Fw32_battery_status’:
> ../../master/src/w32cygwinx.c:116:26: warning: ‘%3.1f’ directive output 
> may be truncated writing between 3 and 312 bytes into a region of size 
> 16 [-Wformat-truncation=]
>      snprintf (buffer, 16, "%3.1f", h);
>                            ^
> ../../master/src/w32cygwinx.c:116:4: note: ‘__builtin_snprintf’ output 
> between 4 and 313 bytes into a destination of size 16
>      snprintf (buffer, 16, "%3.1f", h);
>      ^

Do we really need to use -Wformat-truncation?  Is it a useful warning
switch?  The above sounds like useless noise, because the code
explicitly _asks_ for truncation.  What do people think about this?

> The attached patch avoids the warning.  Is this a reasonable fix, or is
> there a better way?

I think if we keep the switch, a better fix is to do this:

     snprintf (buffer, 16, "%ld", h % 1000000);

     m = seconds_left / 60;
     snprintf (buffer, 16, "%ld", m % 20000);

etc., you get the point.

Thanks.