In some cases, it might be desirable to use the system zlib
Eg. because latest gcc seem to be totally borked when it comes
to multilib, and tries to build a multilib host zlib, when it
is *absolutely* *not* needed: we want mulitlib on the target,
not on the host! Sigh... :-(
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@anciens.enib.fr>