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:	34c19efe9d, e27ff90e17

archivers/pbzip2/Makefile

@@ -16,16 +16,14 @@ PLIST_FILES=	bin/pbzip2 bin/pbunzip2 \
 
 .include <bsd.port.options.mk>
 
-.if ${OPSYS} == FreeBSD && ${OSVERSION} >= 1500000
-USES+=	gmake
-.endif
-
 post-patch:
 	@${REINPLACE_CMD} -e 's,PRI[iu]MAX, & ,g' ${WRKSRC}/pbzip2.cpp \
 		${WRKSRC}/BZ2StreamScanner.cpp
+.if ${OPSYS} == FreeBSD && ${OSVERSION} < 1500053
 # Spell target sources to be understood by both BSD and GNU make(1)
 	@${REINPLACE_CMD} -e '/-O2/s,^,#, ; s,$$^,$$>&,' \
 		${WRKSRC}/Makefile
+.endif
 
 do-install:
 	${INSTALL_PROGRAM} ${WRKSRC}/${PORTNAME} ${STAGEDIR}${PREFIX}/bin/