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/yacc.c

@@ -589,7 +589,7 @@ static const ]b4_int_type_for([b4_translate])[ yytranslate[] =
   ]b4_translate[
 };
 
-#if YYDEBUG
+#if ]b4_api_PREFIX[DEBUG
 /* YYPRHS[YYN] -- Index of the first RHS symbol of rule number YYN in
    YYRHS.  */
 static const ]b4_int_type_for([b4_prhs])[ yyprhs[] =
@@ -610,7 +610,7 @@ static const ]b4_int_type_for([b4_rline])[ yyrline[] =
 };
 #endif
 
-#if YYDEBUG || YYERROR_VERBOSE || ]b4_token_table_flag[
+#if ]b4_api_PREFIX[DEBUG || YYERROR_VERBOSE || ]b4_token_table_flag[
 /* YYTNAME[SYMBOL-NUM] -- String name of the symbol SYMBOL-NUM.
    First, the terminals, then, starting at YYNTOKENS, nonterminals.  */
 static const char *const yytname[] =
@@ -803,7 +803,7 @@ while (YYID (0))
 #endif
 
 /* Enable debugging if requested.  */
-#if YYDEBUG
+#if ]b4_api_PREFIX[DEBUG
 
 # ifndef YYFPRINTF
 #  include <stdio.h> /* INFRINGES ON USER NAME SPACE */
@@ -890,12 +890,12 @@ do {					\
 /* Nonzero means print parse trace.  It is left uninitialized so that
    multiple parsers can coexist.  */
 int yydebug;
-#else /* !YYDEBUG */
+#else /* !]b4_api_PREFIX[DEBUG */
 # define YYDPRINTF(Args)
 # define YY_SYMBOL_PRINT(Title, Type, Value, Location)
 # define YY_STACK_PRINT(Bottom, Top)
 # define YY_REDUCE_PRINT(Rule)
-#endif /* !YYDEBUG */
+#endif /* !]b4_api_PREFIX[DEBUG */
 
 
 /* YYINITDEPTH -- initial size of the parser's stacks.  */
@@ -926,7 +926,7 @@ int yydebug;
    required.  Return 1 if memory is exhausted.  */
 static int
 yy_lac_stack_realloc (YYSIZE_T *yycapacity, YYSIZE_T yyadd,
-#if YYDEBUG
+#if ]b4_api_PREFIX[DEBUG
                       char const *yydebug_prefix,
                       char const *yydebug_suffix,
 #endif
@@ -1032,7 +1032,7 @@ do {                                                             \
    the parser stacks to try to find a new initial context in which the
    current lookahead is syntactically acceptable.  If it fails to find
    such a context, it discards the lookahead.  */
-#if YYDEBUG
+#if ]b4_api_PREFIX[DEBUG
 # define YY_LAC_DISCARD(Event)                                           \
 do {                                                                     \
   if (yy_lac_established)                                                \
@@ -1135,7 +1135,7 @@ yy_lac (yytype_int16 *yyesa, yytype_int16 **yyes,
         else
           {
             if (yy_lac_stack_realloc (yyes_capacity, 1,
-#if YYDEBUG
+#if ]b4_api_PREFIX[DEBUG
                                       " (", ")",
 #endif
                                       yyes, yyesa, &yyesp, yyes_prev))
@@ -1342,7 +1342,7 @@ yysyntax_error (YYSIZE_T *yymsg_alloc, char **yymsg,
                 yysize = yysize1;
               }
         }]b4_lac_if([[
-# if YYDEBUG
+# if ]b4_api_PREFIX[DEBUG
       else if (yydebug)
         YYFPRINTF (stderr, "No expected tokens.\n");
 # endif]])[