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]

tests/local.at

@@ -752,7 +752,7 @@ m4_define([AT_BISON_CHECK_XML],
   # Don't combine these Bison invocations since we want to be sure that
   # --report=all isn't required to get the full XML file.
   AT_BISON_CHECK_([[--report=all --report-file=xml-tests/test.output \
-             --graph=xml-tests/test.dot ]]AT_BISON_ARGS,
+             --graph=xml-tests/test.gv ]]AT_BISON_ARGS,
            [[0]], [ignore], [ignore])
   AT_BISON_CHECK_([[--xml=xml-tests/test.xml ]]AT_BISON_ARGS,
            [[0]], [ignore], [ignore])
@@ -761,7 +761,7 @@ m4_define([AT_BISON_CHECK_XML],
   AT_CHECK([[$XSLTPROC \
              `]]AT_QUELL_VALGRIND[[ bison --print-datadir`/xslt/xml2text.xsl \
              xml-tests/test.xml]], [[0]], [expout])
-  [sort xml-tests/test.dot > expout]
+  [sort xml-tests/test.gv > expout]
   AT_CHECK([[$XSLTPROC \
              `]]AT_QUELL_VALGRIND[[ bison --print-datadir`/xslt/xml2dot.xsl \
              xml-tests/test.xml | sort]], [[0]], [expout])

tests/output.at

@@ -76,6 +76,8 @@ AT_CHECK_OUTPUT([foo.y], [], [-dv -o foo.tab.c],
 
 AT_CHECK_OUTPUT([foo.y], [], [-dv -g --xml -y],
                 [y.dot y.output y.tab.c y.tab.h y.xml])
+AT_CHECK_OUTPUT([foo.y], [%require "3.4"], [-dv -g --xml -y],
+                [y.gv y.output y.tab.c y.tab.h y.xml])
 # With '-o y.tab.c', we expect 'y.output' etc. (for compatibility with Yacc).
 AT_CHECK_OUTPUT([foo.y], [], [-dv -g --xml -o y.tab.c],
                 [y.dot y.output y.tab.c y.tab.h y.xml])