Config: use sed for substituting variables

Substitutions generated by configure don't resolve prefixes, so
cannot be used for paths. Config already had localedir, and next
commit will need libexecdir and sysconfdir, so just bite the bullet
and move to sed.

https://bugzilla.gnome.org/show_bug.cgi?id=658484

js/misc/config.js.in

@@ -14,3 +14,6 @@ const SHELL_SYSTEM_CA_FILE = '@SHELL_SYSTEM_CA_FILE@';
 const GETTEXT_PACKAGE = '@GETTEXT_PACKAGE@';
 /* locale dir */
 const LOCALEDIR = '@datadir@/locale';
+/* other standard directories */
+const LIBEXECDIR = '@libexecdir@';
+const SYSCONFDIR = '@sysconfdir@';