rsync: conditionally enable included zlib

rsync, by default, includes a patched zlib within its source tarball. The patch enables a better compression ratio for rsync
streams. However, the client and server need the same featureset in their zlib in order to understand the compression stream,
or the server will abort the transfer:

2014/09/02 14:05:46 [1610] connect from yggdrasil.lan (192.168.1.250)
2014/09/02 21:05:46 [1610] rsync on gentoo-portage/metadata/timestamp.chk from yggdrasil.lan (192.168.1.250)
2014/09/02 21:05:46 [1610] rsync: This rsync lacks old-style --compress due to its external zlib.  Try -zz.

Older versions have no understanding of the meaning of "-zz".

This patch conditionally enables the patched zlib within rsync so as to remain compatible with earlier versions of rsync or
current versions that are using default settings.

Signed-off-by: Ian Leonard <antonlacon@gmail.com>