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-src/ebrowse.c

@@ -3059,8 +3059,7 @@ class_definition (struct sym *containing, int tag, int flags, int nested)
                  MATCH until we see something like `;' or `{'.  */
               while (!LOOKING_AT3 (';', YYEOF, '{'))
                 MATCH ();
-	      done = 1;
-
+	      FALLTHROUGH;
             case '{':
               done = 1;
 	      break;
@@ -3184,7 +3183,7 @@ declaration (int flags)
 	      free (id);
 	      return;
 	    }
-
+	  FALLTHROUGH;
         case '=':
           /* Assumed to be the start of an initialization in this
 	     context.  */