Use system zlib if available

Only fall back to miniz if we don't find zlib on the system. Distro
packagers like this much more, and we use code that has seen much
more testing. Incidentally, we've already had one case where miniz was
really slow on a 32 bit box.

Still keeping miniz as fallback for the rare platform/build box that
doesn't ship zlib, since waiting a few seconds for compressing the
backlog is still probably better than transmitting a few MB of extra
data...