rsync: enable selecting bundled or system 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 allows the builder to choose whether to use the bundled or system
zlib. It defautls to using the system zlib, as is the default before this
patch.

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