glr.c, yacc.c: declare yydebug in the header.

* data/c.m4 (b4_declare_yydebug): New.
* data/glr.c, data/yacc.c (b4_shared_declarations): Use it.
Remove the corresponding code from the parser body.
* NEWS: Doc this.

data/c.m4

@@ -567,3 +567,15 @@ typedef struct YYLTYPE
 # define YYLTYPE_IS_TRIVIAL 1
 #endif]])
 ])
+
+# b4_declare_yydebug
+# ------------------
+m4_define([b4_declare_yydebug],
+[[/* Enabling traces.  */
+#ifndef YYDEBUG
+# define YYDEBUG ]b4_debug_flag[
+#endif
+#if YYDEBUG
+extern int ]b4_prefix[debug;
+#endif][]dnl
+])