Under some circumstances the installed / system version of zlib may not
be desirable due to being too old or buggy.  This patch adds the option
`USE_BUNDLED_ZLIB` that will cause the bundled version of zlib to be
used.

We may also want to add similar functionality to allow the user to
select other bundled 3rd-party dependencies instead of using the system
versions.

/cc @pks-t @ethomson