Remake the way dynamic zlib is loaded

Remake the way dynamic zlib is loaded

Instead of absolute hard coding of the libz library name, have it use
the macro LIBZ, which is set to defaults we know in case it's
undefined.

This allows our configuration to set something that's sane on current
or older platforms, and allows the user to override it by defining
LIBZ themselves.

Reviewed-by: Matt Caswell <matt@openssl.org>