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/getargs.c

@@ -49,7 +49,7 @@
 #include "quote.h"
 #include "uniqstr.h"
 
-bool debug_flag;
+bool debug;
 bool defines_flag;
 bool graph_flag;
 bool xml_flag;
@@ -664,7 +664,7 @@ getargs (int argc, char *argv[])
 	break;
 
       case 't':
-	debug_flag = true;
+	debug = true;
 	break;
 
       case 'v':