Add VCL 4.0 marker in default.vcl.

Fixes: #1413

etc/Makefile.am

@@ -6,7 +6,7 @@ DISTCLEANFILES = default.vcl
 dist_varnishconf_DATA = default.vcl
 
 default.vcl:	$(top_srcdir)/bin/varnishd/default.vcl
-	( printf "This is a basic VCL configuration file for varnish.  See the vcl(7)\n\
+	( printf "vcl 4.0;\nThis is a basic VCL configuration file for varnish.  See the vcl(7)\n\
 man page for details on VCL syntax and semantics.\n\
 \n\
 Default backend definition.  Set this to point to your content\n\