Fix race conditions handling selection clear events on Haiku

Fix race conditions handling selection clear events on Haiku

* src/haiku_select.cc (be_handle_clipboard_changed_message):
Include current clipboard count.
(be_selection_outdated_p): New function.
* src/haikuselect.c (haiku_handle_selection_clear): Ignore
outdated events.
(haiku_selection_disowned): New argument `count'.  Include it in
the timestamp field of the selection clear event.
* src/haikuselect.h: Update prototypes.
* src/systime.h: Define `Time' to an appropriate value on Haiku.