Remove dependency on GNU make(1).

It turns out that GNU make(1) is only required because their make(1) spells
target dependencies as $^ while ours use $>. This change makes the pbzip2's
Makefile both GNU and BSD make(1) friendly at the same time.

This is required to be able to use `archivers/pbzip2' port as a PKG_DEPEND.

Approved by: farrokhi (maintainer)