Clarity around zlib usage

[grunt-lucas]

It seems that libpng is dependent on zlib. MacOS ships with a default zlib, but Porytiles uses the latest via homebrew. This should be made clear in the install instructions, or I should change MacOS Porytiles to use the default zlib. Note that homebrew libpng declares its zlib dependency to be the MacOS shipped zlib: https://github.com/Homebrew/homebrew-core/blob/master/Formula/lib/libpng.rb

I think I added it explicitly because for linux builds, I link statically. And hence libpng needs zlib installed to link properly. However, for the dynamically linked MacOS builds, I don't actually need to specify zlib specifically, since it is specified as a private requirement in libpng. I should clean up the makefile and build scripts accordingly so it's not so confusing.