graph: prefer *.gv to *.dot

Reported by Hans Åberg.
https://lists.gnu.org/archive/html/help-bison/2019-02/msg00064.html

* src/files.c (spec_graph_file): Use `*.gv` when 3.4 or better,
otherwise `*.dot`.
* src/parse-gram.y (handle_require): Pretend we are already 3.4.
* doc/bison.texi: Adjust.
* tests/local.at, tests/output.at: Exercise this.

src/files.c

@@ -342,7 +342,8 @@ compute_output_file_names (void)
   if (graph_flag)
     {
       if (! spec_graph_file)
-        spec_graph_file = concat2 (all_but_tab_ext, ".dot");
+        spec_graph_file = concat2 (all_but_tab_ext,
+                                   304 <= required_version ? ".gv" : ".dot");
       output_file_name_check (&spec_graph_file, false);
     }