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.  */

lib-src/etags.c

@@ -1157,7 +1157,7 @@ main (int argc, char **argv)
       case 'c':
 	/* Backward compatibility: support obsolete --ignore-case-regexp. */
 	optarg = concat (optarg, "i", ""); /* memory leak here */
-	/* FALLTHRU */
+	FALLTHROUGH;
       case 'r':
 	argbuffer[current_arg].arg_type = at_regexp;
 	argbuffer[current_arg].what = optarg;
@@ -1192,7 +1192,7 @@ main (int argc, char **argv)
       case 't': typedefs = true;				break;
       case 'T': typedefs = typedefs_or_cplusplus = true;	break;
       case 'u': update = true;					break;
-      case 'v': vgrind_style = true;			  /*FALLTHRU*/
+      case 'v': vgrind_style = true;				FALLTHROUGH;
       case 'x': cxref_style = true;				break;
       case 'w': no_warnings = true;				break;
       default:
@@ -2564,7 +2564,7 @@ hash (const char *str, int len)
     {
       default:
         hval += asso_values[(unsigned char) str[2]];
-      /*FALLTHROUGH*/
+	FALLTHROUGH;
       case 2:
         hval += asso_values[(unsigned char) str[1]];
         break;
@@ -3013,7 +3013,7 @@ consider_token (char *str, int len, int c, int *c_extp,
 	 *c_extp = (*c_extp | C_PLPL) & ~C_AUTO;
        if (toktype == st_C_template)
 	 break;
-       /* FALLTHRU */
+       FALLTHROUGH;
      case st_C_struct:
      case st_C_enum:
        if (parlev == 0
@@ -3176,7 +3176,7 @@ consider_token (char *str, int len, int c, int *c_extp,
 	     default:
 	       break;
 	     }
-	  /* FALLTHRU */
+	   FALLTHROUGH;
 	  case fvnameseen:
 	  if (len >= 10 && strneq (str+len-10, "::operator", 10))
 	    {
@@ -3387,7 +3387,7 @@ C_entries (int c_ext, FILE *inf)
 	    case '\0':
 	      /* Hmmm, something went wrong. */
 	      CNL ();
-	      /* FALLTHRU */
+	      FALLTHROUGH;
 	    case '\'':
 	      inchar = false;
 	      break;
@@ -3828,7 +3828,7 @@ C_entries (int c_ext, FILE *inf)
 		      || (members
 			  && plainc && instruct))
 		    make_C_tag (true);  /* a function */
-		  /* FALLTHRU */
+		  FALLTHROUGH;
 		default:
 		  fvextern = false;
 		  fvdef = fvnone;
@@ -3838,7 +3838,7 @@ C_entries (int c_ext, FILE *inf)
 		  else
 		    token.valid = false;
 		} /* switch (fvdef) */
-	      /* FALLTHRU */
+	      FALLTHROUGH;
 	    default:
 	      if (!instruct)
 		typdef = tnone;
@@ -3926,7 +3926,7 @@ C_entries (int c_ext, FILE *inf)
 		      || (globals && bracelev == 0
 			  && (!fvextern || declarations)))
 		    make_C_tag (false); /* a variable */
-		  /* FALLTHRU */
+		  FALLTHROUGH;
 		default:
 		  fvdef = fvnone;
 		}
@@ -3959,7 +3959,7 @@ C_entries (int c_ext, FILE *inf)
 		  fvdef = fignore;
 		  break;
 		}
-	      /* FALLTHRU */
+	      FALLTHROUGH;
 	    case foperator:
 	      fvdef = fstartlist;
 	      break;
@@ -4049,7 +4049,7 @@ C_entries (int c_ext, FILE *inf)
 		    }
 		}
 	      make_C_tag (true);    /* a function */
-	      /* FALLTHRU */
+	      FALLTHROUGH;
 	    case fignore:
 	      fvdef = fvnone;
 	      break;
@@ -4142,7 +4142,7 @@ C_entries (int c_ext, FILE *inf)
 	      if ((members && bracelev == 1)
 		  || (globals && bracelev == 0 && (!fvextern || declarations)))
 		make_C_tag (false); /* a variable */
-	      /* FALLTHRU */
+	      FALLTHROUGH;
 	    default:
 	      fvdef = vignore;
 	    }
@@ -4169,7 +4169,7 @@ C_entries (int c_ext, FILE *inf)
 	      objdef = omethodsign;
 	      break;
 	    }
-	  /* FALLTHRU */
+	  FALLTHROUGH;
 	resetfvdef:
 	case '#': case '~': case '&': case '%': case '/':
 	case '|': case '^': case '!': case '.': case '?':
@@ -6354,7 +6354,7 @@ add_regex (char *regexp_pattern, language *lang)
 	break;
       case 's':
 	single_line = true;
-	/* FALLTHRU */
+	FALLTHROUGH;
       case 'm':
 	multi_line = true;
 	need_filebuf = true;