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. Older versions have
no understanding of the new compression standard used.

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>