maint: be compilable with GCC 4.0

The "shadows a global declaration" warning in GCC 4.0 was a bit
annoying.  It does not like that a type name be used in a prototype of
a function (not the implementation, just the declaration):

  In file included from src/LR0.c:38:
  src/reader.h:56: warning: declaration of 'named_ref' shadows a
                            global declaration
  src/named-ref.h:35: warning: shadowed declaration is here

It does not like either when a global variable name is used in a
prototype.  Flex 2.5.37 generates this prototype:

  void gram_set_debug (int debug_flag  );

* src/getargs.h, src/getargs.c (debug_flag): Rename as...
(debug): this.
Adjust dependencies.
* src/reader.h: Don't use "named_ref" as a formal argument name.

src/parse-gram.y

@@ -252,7 +252,7 @@ prologue_declaration:
                         plain_code.code, @1);
       code_scanner_last_string_free ();
     }
-| "%debug"                         { debug_flag = true; }
+| "%debug"                         { debug = true; }
 | "%define" variable content.opt
     {
       muscle_percent_define_insert ($2, @2, $3,