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);
     ^

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


In GNU Emacs 27.0.50 (build 6, x86_64-unknown-cygwin, GTK+ Version 3.22.28)
  of 2018-07-17 built on moufang
Repository revision: 04599bb1b219b236356ba3393a23e1c1dd8c541b
Windowing system distributor 'The Cygwin/X Project', version 11.0.12000000