Avoid compiler warnings on MS-Windows with GCC 6 and 7

Avoid compiler warnings on MS-Windows with GCC 6 and 7

* src/w32font.c (SUBRANGE): Use unsigned arithmetic for
bit-shifting, to avoid compiler warnings.
(w32font_text_extents): Tell GCC NGLYPHS is non-negative, to avoid
a warning.  For details of the warning, see
http://lists.gnu.org/archive/html/emacs-devel/2017-09/msg00093.html.
* src/term.c (keys) [WINDOWSNT]: Don't define, as it is not used
in that build.
* src/sound.c (sound_perror): Ifdef away on WINDOWSNT, as this
function is not used in that build.

* configure.ac: Disable -Wsuggest-attribute=format on MS-Windows.