how to detect if gtk2 module is available

[brian <bmcandrews gmail com>]

I have a separate perl module that sets a Tk::fileevent watch if the
user passes in a Tk window.  Now I want the client to be able to set
hook up their own events to  Gtk2 via Gtk2::Helper->add_watch.  This
is simple enough, but the problem I have is that the Gtk2 module is
not installed everywhere, since a lot of our stuff is still on
perl-5.6.1.  However, clients do have access to 5.8.0.  How can I
conditionally test for the availability of Gtk2 inside a module?  I
still want the 5.6.1 users to have access to this module, but I want
our 5.8.0 to be able use Gtk2 with this module.

Thanks,
Brian