Merge with gnulib, pacifying GCC 7

This incorporates:
2017-05-16 manywarnings: update for GCC 7
2017-05-15 sys_select: Avoid "was expanded before it was required"
* configure.ac (nw): Suppress GCC 7’s new -Wduplicated-branches and
-Wformat-overflow=2 options, due to too many false alarms.
* doc/misc/texinfo.tex, lib/strftime.c, m4/manywarnings.m4:
Copy from gnulib.
* m4/gnulib-comp.m4: Regenerate.
* src/coding.c (decode_coding_iso_2022):
Fix bug uncovered by -Wimplicit-fallthrough.
* src/conf_post.h (FALLTHROUGH): New macro.
Use it to mark all switch cases that fall through.
* src/editfns.c (styled_format): Use !, not ~, on bool.
* src/gtkutil.c (xg_check_special_colors):
When using sprintf, don’t trust Gtk to output colors in [0, 1] range.
(xg_update_scrollbar_pos): Avoid use of possibly-uninitialized bool;
this bug was actually caught by Clang.
* src/search.c (boyer_moore):
Tell GCC that CHAR_BASE, if nonzero, must be a non-ASCII character.
* src/xterm.c (x_draw_glyphless_glyph_string_foreground):
Tell GCC that glyph->u.glyphless.ch must be a character.

lib/strftime.c

@@ -68,6 +68,14 @@ extern char *tzname[];
 #include <string.h>
 #include <stdbool.h>
 
+#ifndef FALLTHROUGH
+# if __GNUC__ < 7
+#  define FALLTHROUGH ((void) 0)
+# else
+#  define FALLTHROUGH __attribute__ ((__fallthrough__))
+# endif
+#endif
+
 #ifdef COMPILE_WIDE
 # include <endian.h>
 # define CHAR_T wchar_t
@@ -1138,8 +1146,7 @@ __strftime_internal (STREAM_OR_CHAR_T *s, STRFTIME_ARG (size_t maxsize)
 #ifndef _NL_CURRENT
           format_char = L_('p');
 #endif
-          /* FALLTHROUGH */
-
+          FALLTHROUGH;
         case L_('p'):
           if (change_case)
             {
@@ -1474,7 +1481,7 @@ __strftime_internal (STREAM_OR_CHAR_T *s, STRFTIME_ARG (size_t maxsize)
 
         case L_('\0'):          /* GNU extension: % at end of format.  */
             --f;
-            /* Fall through.  */
+            FALLTHROUGH;
         default:
           /* Unknown format; output the format, including the '%',
              since this is most likely the right thing to do if a