bug#53894: 27.2; Clipboard copy -> C-y -> M-y -> Same clipboard copy does not push to kill ring

[Po Lu]

Ignacio Casso <ignaciocasso@hotmail.com> writes:

> You are right that it is a dangerous assumption, and it's also hard to
> test and probably unnecessary. So I have changed the code to check the
> window-system variable instead and only use timestamps for X and haiku

Haiku doesn't support timestamps, only `gui-backend-selection-owner-p'
is reliable.  But I can expose the selection counter if you think that's
a good idea.

Thanks.