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/glr.cc

@@ -127,7 +127,7 @@ m4_pushdef([b4_parse_param], m4_defn([b4_parse_param_orig]))dnl
 [  /// 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[
@@ -144,7 +144,7 @@ m4_pushdef([b4_parse_param], m4_defn([b4_parse_param_orig]))dnl
     return ::yyparse (*this]b4_user_args[);
   }
 
-#if YYDEBUG
+#if ]b4_api_PREFIX[DEBUG
   /*--------------------.
   | Print this symbol.  |
   `--------------------*/
@@ -239,10 +239,7 @@ b4_copyright([Skeleton interface for Bison GLR parsers in C++],
 ]b4_percent_define_ifdef([[location_type]], [],
                          [[#include "location.hh"]])[
 
-/* Enabling traces.  */
-#ifndef YYDEBUG
-# define YYDEBUG ]b4_debug_flag[
-#endif
+]b4_YYDEBUG_define[
 
 /* YYLLOC_DEFAULT -- Set CURRENT to span from RHS[1] to RHS[N].
    If N is 0, then set CURRENT to the empty location which ends
@@ -321,7 +318,7 @@ b4_user_stype
     virtual void error (const location_type& loc, const std::string& msg);
   private:
 
-#if YYDEBUG
+# if ]b4_api_PREFIX[DEBUG
   public:
     /// \brief Report a symbol value on the debug stream.
     /// \param yytype       The token type.
@@ -341,7 +338,7 @@ b4_user_stype
     /* Debugging.  */
     int yydebug_;
     std::ostream* yycdebug_;
-#endif
+# endif
 
 ]b4_parse_param_vars[
   };