traces: display the Bison version

* src/main.c (main): When traces are enabled, display the Bison
version.
* tests/conflicts.at, tests/report.at, tests/sets.at:
Use AT_PACKAGE_VERSION (for package.m4) instead of post-processing the
output.

src/main.c

@@ -94,6 +94,9 @@ main (int argc, char *argv[])
 
   getargs (argc, argv);
 
+  if (trace_flag)
+    fprintf (stderr, "bison (GNU Bison) %s\n", VERSION);
+
   timevar_enabled = trace_flag & trace_time;
   timevar_init ();
   timevar_start (tv_total);