notify_desktop plugin: Add compatibility code for libnotify 0.7.

notify_desktop plugin: Add compatibility code for libnotify 0.7.

The current code in src/notify_desktop.c is not compatible with the new
libnotify 0.7 API

This:

  notification = notify_notification_new (summary, n->message, NULL, NULL);

Should now be:

  notification = notify_notification_new (summary, n->message, NULL;

As in, one argument less for notify_notification_new.

But we can't just remove it or it'll break compability with libnotify
0.4.x and 0.5.x.

This piece of code sets dummy NOTIFY_CHECK_VERSION for libnotify-0.4.x,
because NOTIFY_CHECK_VERSION was added only in 0.5.x:

  #ifndef NOTIFY_CHECK_VERSION
  #define NOTIFY_CHECK_VERSION(x,y,z) 0
  #endif

Then we can freely use NOTIFY_CHECK_VERSION, and it will work with all
of 0.4, 0.5 and 0.7 series:

  #if NOTIFY_CHECK_VERSION (0, 7, 0)
  do something cool
  #endif

I'm attaching the working patch.  It's been tested with all of the
mentioned versions, as well as it's in active use at Gentoo Linux.

Thanks, Samuli

Signed-off-by: Florian Forster <octo@huhu.verplant.org>