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.

tests/.gitignore

@@ -1,8 +1,6 @@
-/*.dot
 /*.output
 /atconfig
 /atlocal
-/autom4te.cache
 /bison
 /calc
 /calc.[chy]