networkAgent: Pick up VPN service dir from pkg-config

It turns out that NetworkManager does export the directory as pkg-config
variable after all, so use that instead of building the path ourselves
from the prefix.

https://bugzilla.gnome.org/show_bug.cgi?id=789811

js/misc/config.js.in

@@ -14,6 +14,6 @@ var GETTEXT_PACKAGE = '@GETTEXT_PACKAGE@';
 var LOCALEDIR = '@datadir@/locale';
 /* other standard directories */
 var LIBEXECDIR = '@libexecdir@';
-var NMPREFIXDIR = '@nmprefixdir@';
+var VPNDIR = '@vpndir@';
 /* g-i package versions */
 var LIBMUTTER_API_VERSION = '@LIBMUTTER_API_VERSION@'