archivers/pbzip2: rework previous naive and inaccurate build fix

Back in 2017 I've untied this port from GNU make(1) so I could use
it as a PKG_DEPENDS, by spelling target's prerequisites in both GNU
and BSD make(1) syntax, which seemed like a nifty trick at the time.

It worked well until the import of bmake-20250707 in July which had
added POSIX $^ support, so now we were listing prerequisites twice
without separating them with space, in turn causing the breakage.

While we could probably just use the old BSD $> (or even .ALLSRC),
tentatively keep existing hack, guarded by the closest OSVERSION, so
it would go away naturally when all supported FreeBSD versions would
understand $^ (unconditional replacing would have to stay forever
because upstreaming old BSD $> when GNU, POSIX, and new BSD use $^
is unfeasible).

PR: 290004
Fixes: 34c19efe9dc6, e27ff90e17f2