api.prefix: also rename YYDEBUG.

The testsuite in master has shown weird errors for the "Mulitple
Parsers" tests: the caller of p5.parse() received some apparently
random value, while tracing p5.parse() showed that the function was
consistently returning 0.

It happens when mixing several parser headers, some generated without
%debug, others with.  In particular the C++ parser was generated with
%debug, i.e., with:

  #ifndef YYDEBUG
  # define YYDEBUG 1
  #endif

and compiled separatedly.  Yet, its header was included after the one
of another parser, this time without %debug, i.e., with

  #ifndef YYDEBUG
  # define YYDEBUG 0
  #endif

in its header.  As a result, the parser was compiled with YYDEBUG set,
but its header was used without.  Since the layout of the objects are
then completely different, boom.

Therefore, do not change the value of YYDEBUG.  Rather, use it as a
default value for <API.PREFIX>DEBUG.

* data/c.m4 (b4_YYDEBUG_define): New.
(b4_declare_yydebug): Rename as...
(b4_yydebug_declare): this, for consistency.
* data/glr.c, data/glr.cc, data/lalr1.cc, data/yacc.c: Use it.
* NEWS: Document it.

data/lalr1.cc

@@ -57,10 +57,7 @@ b4_copyright([Skeleton interface for Bison LALR(1) parsers in C++],
 
 ]b4_null_define[
 
-/* Enabling traces.  */
-#ifndef YYDEBUG
-# define YYDEBUG ]b4_debug_flag[
-#endif
+]b4_YYDEBUG_define[
 
 ]b4_namespace_open[
 
@@ -100,7 +97,7 @@ b4_user_stype
     /// \returns  0 iff parsing succeeded.
     virtual int parse ();
 
-#if YYDEBUG
+#if ]b4_api_PREFIX[DEBUG
     /// The current debugging stream.
     std::ostream& debug_stream () const;
     /// Set the current debugging stream.
@@ -125,7 +122,7 @@ b4_user_stype
     /// \param tok     the lookahead token.
     virtual std::string yysyntax_error_ (int yystate, int tok);
 
-#if YYDEBUG
+#if ]b4_api_PREFIX[DEBUG
     /// \brief Report a symbol value on the debug stream.
     /// \param yytype       The token type.
     /// \param yyvaluep     Its semantic value.
@@ -203,10 +200,10 @@ b4_user_stype
     /// Convert the symbol name \a n to a form suitable for a diagnostic.
     static std::string yytnamerr_ (const char *n);])[
 
-]b4_token_table_if([], [[#if YYDEBUG]])[
+]b4_token_table_if([], [[#if ]b4_api_PREFIX[DEBUG]])[
     /// For a symbol, its name in clear.
     static const char* const yytname_[];
-]b4_token_table_if([[#if YYDEBUG]])[
+]b4_token_table_if([[#if ]b4_api_PREFIX[DEBUG]])[
     /// A type to store symbol numbers and -1.
     typedef ]b4_int_type_for([b4_rhs])[ rhs_number_type;
     /// A `-1'-separated list of the rules' RHS.
@@ -325,7 +322,7 @@ b4_percent_code_get[]dnl
 #define YYUSE(e) ((void) (e))
 
 /* Enable debugging if requested.  */
-#if YYDEBUG
+#if ]b4_api_PREFIX[DEBUG
 
 /* A pseudo ostream that takes yydebug_ into account.  */
 # define YYCDEBUG if (yydebug_) (*yycdebug_)
@@ -352,14 +349,14 @@ do {					\
     yystack_print_ ();			\
 } while (false)
 
-#else /* !YYDEBUG */
+#else /* !]b4_api_PREFIX[DEBUG */
 
 # define YYCDEBUG if (false) std::cerr
 # define YY_SYMBOL_PRINT(Title, Type, Value, Location)
 # define YY_REDUCE_PRINT(Rule)
 # define YY_STACK_PRINT()
 
-#endif /* !YYDEBUG */
+#endif /* !]b4_api_PREFIX[DEBUG */
 
 #define yyerrok		(yyerrstatus_ = 0)
 #define yyclearin	(yychar = yyempty_)
@@ -412,7 +409,7 @@ do {					\
   /// Build a parser object.
   ]b4_parser_class_name::b4_parser_class_name[ (]b4_parse_param_decl[)]m4_ifset([b4_parse_param], [
     :])[
-#if YYDEBUG
+#if ]b4_api_PREFIX[DEBUG
     ]m4_ifset([b4_parse_param], [  ], [ :])[yydebug_ (false),
       yycdebug_ (&std::cerr)]m4_ifset([b4_parse_param], [,])[
 #endif]b4_parse_param_cons[
@@ -423,7 +420,7 @@ do {					\
   {
   }
 
-#if YYDEBUG
+#if ]b4_api_PREFIX[DEBUG
   /*--------------------------------.
   | Print this symbol on YYOUTPUT.  |
   `--------------------------------*/
@@ -484,7 +481,7 @@ do {					\
     yylocation_stack_.pop (n);
   }
 
-#if YYDEBUG
+#if ]b4_api_PREFIX[DEBUG
   std::ostream&
   ]b4_parser_class_name[::debug_stream () const
   {
@@ -1012,7 +1009,7 @@ b4_error_verbose_if([int yystate, int yytoken],
     ]b4_stos[
   };
 
-#if YYDEBUG
+#if ]b4_api_PREFIX[DEBUG
   /* TOKEN_NUMBER_[YYLEX-NUM] -- Internal symbol number corresponding
      to YYLEX-NUM.  */
   const ]b4_int_type_for([b4_toknum])[
@@ -1036,7 +1033,7 @@ b4_error_verbose_if([int yystate, int yytoken],
     ]b4_r2[
   };
 
-]b4_token_table_if([], [[#if YYDEBUG]])[
+]b4_token_table_if([], [[#if ]b4_api_PREFIX[DEBUG]])[
   /* YYTNAME[SYMBOL-NUM] -- String name of the symbol SYMBOL-NUM.
      First, the terminals, then, starting at \a yyntokens_, nonterminals.  */
   const char*
@@ -1045,7 +1042,7 @@ b4_error_verbose_if([int yystate, int yytoken],
     ]b4_tname[
   };
 
-]b4_token_table_if([[#if YYDEBUG]])[
+]b4_token_table_if([[#if ]b4_api_PREFIX[DEBUG]])[
   /* YYRHS -- A `-1'-separated list of the rules' RHS.  */
   const ]b4_parser_class_name[::rhs_number_type
   ]b4_parser_class_name[::yyrhs_[] =
@@ -1095,7 +1092,7 @@ b4_error_verbose_if([int yystate, int yytoken],
 		       &]b4_rhs_value(yynrhs, yyi + 1)[,
 		       &]b4_rhs_location(yynrhs, yyi + 1)[);
   }
-#endif // YYDEBUG
+#endif // ]b4_api_PREFIX[DEBUG
 
   /* YYTRANSLATE(YYLEX) -- Bison symbol number corresponding to YYLEX.  */
   ]b4_parser_class_name[::token_number_type