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.

src/parse-gram.y

@@ -143,6 +143,7 @@
 /* Define the tokens together with their human representation.  */
 %token GRAM_EOF 0 "end of file"
 %token STRING     "string"
+       TSTRING    "translatable string"
 
 %token PERCENT_TOKEN       "%token"
 %token PERCENT_NTERM       "%nterm"
@@ -216,8 +217,8 @@
 %type <unsigned char> CHAR
 %printer { fputs (char_name ($$), yyo); } <unsigned char>
 
-%type <char*> "{...}" "%?{...}" "%{...%}" EPILOGUE STRING
-%printer { fputs ($$, yyo); }  <char*>
+%type <char*> "{...}" "%?{...}" "%{...%}" EPILOGUE STRING TSTRING
+%printer { fputs ($$, yyo); } <char*>
 
 %type <uniqstr>
   BRACKETED_ID ID ID_COLON
@@ -532,7 +533,7 @@ token_decl.1:
 
 // One symbol declaration for %token or %nterm.
 token_decl:
-  id int.opt[num] string_as_id.opt[alias]
+  id int.opt[num] alias
     {
       $$ = $id;
       symbol_class_set ($id, current_class, @id, true);
@@ -549,6 +550,19 @@ int.opt:
 | INT
 ;
 
+%type <symbol*> alias;
+alias:
+  %empty         { $$ = NULL; }
+| string_as_id   { $$ = $1; }
+| TSTRING
+    {
+      $$ = symbol_get (quotearg_style (c_quoting_style, $1), @1);
+      symbol_class_set ($$, token_sym, @1, false);
+      $$->translatable = true;
+    }
+;
+
+
 /*-------------------------------------.
 | token_decls_for_prec (%left, etc.).  |
 `-------------------------------------*/
@@ -782,12 +796,6 @@ string_as_id:
     }
 ;
 
-%type <symbol*> string_as_id.opt;
-string_as_id.opt:
-  %empty             { $$ = NULL; }
-| string_as_id
-;
-
 epilogue.opt:
   %empty
 | "%%" EPILOGUE