To my knowledge zlib doesn't have a public bug tracker, that's why I've notified the problem to the mantainers via email.
The MacPorts bug about the same issue is on https://trac.macports.org/ticket/39523

Again, this is a very well known issue that it's very obvious when you build zlib on OS X: instead of a dylib, a .so library is created instead when "cc" is found to be the default compiler.