Fix path to suse-bzip2's libbz2 dynamic library on Darwin.

On Darwin (OS X 10.8.4), $(PREFIX)/lib/pristine-tar/suse-bzip2/bzip2 did not find its libbz2 dynamic library, as the pit/suse-bzip2 Makefile passed $(PREFIX)/lib/zgz/suse-bzip2/libbz2.1.0.4.dylib as the install name to the linker instead of $(PREFIX)/lib/pristine-tar/suse-bzip2/libbz2.1.0.4.dylib.