zlib: pkgconfig support

I’m sure there’s a legitimate reason we haven’t done this before, and
there’s some conversation I’ve missed somewhere, but I’d love to get
this done and be able to stop doing the ZLIB CFLAG/LDFLAG dance so
often. This is a rough draft, I doubt the zlib version is actually
identical for the last 3 OS versions, although it could have been. But
if we can get this done, I’d be super up for it.