parsers: support translatable token aliases

In addition to

    %token NUM "number"

accept

    %token NUM _("number")

in which case the token will be translated in error messages.
Do not use _() in the output if there are no translatable tokens.

* src/symtab.h, src/symtab.c (symbol): Add a 'translatable' member.
* src/parse-gram.y (TSTRING): New token.
(string_as_id.opt): Replace with...
(alias): this.
Use it.
* src/scan-gram.l (SC_ESCAPED_TSTRING): New start conditions, to match
TSTRINGs.
* src/output.c (prepare_symbols): Define b4_translatable if there are
translatable strings.

* data/skeletons/glr.c, data/skeletons/lalr1.cc,
* data/skeletons/yacc.c (yytnamerr): Receive b4_translatable, and use it.

data/skeletons/yacc.c

@@ -631,8 +631,17 @@ yysymbol_name (int yysymbol)
   static const char *const yy_sname[] =
   {
   ]b4_symbol_names[
+  };]m4_ifdef([b4_translatable], [[
+  /* YYTRANSLATABLE[SYMBOL-NUM] -- Whether YYTNAME[SYMBOL-NUM] is
+     internationalizable.  */
+  static ]b4_int_type_for([b4_translate])[ yytranslatable[] =
+  {
+  ]b4_translatable[
   };
-  return yy_sname[yysymbol];
+  return (yysymbol < YYNTOKENS && yytranslatable[yysymbol]
+          ? _(yy_sname[yysymbol])
+          : yy_sname[yysymbol]);]], [[
+  return yy_sname[yysymbol];]])[
 }]])[
 #endif
 
@@ -1248,7 +1257,6 @@ yytnamerr (char *yyres, const char *yystr)
     {
       YYPTRDIFF_T yyn = 0;
       char const *yyp = yystr;
-
       for (;;)
         switch (*++yyp)
           {