Restore the zlib / zlib-dynamic logic

The proper logic is that both zlib and zlib-dynamic are disabled by
default and that enabling zlib-dynamic would enable zlib.  Somewhere
along the way, the logic got changed, zlib-dynamic was enabled by
default and zlib didn't get automatically enabled.

This change restores the original logic.

Reviewed-by: Rich Salz <rsalz@openssl.org>