Use the official archive of zlib

[puremourning]

zlib.net only has the latest version in the "root". The latest version is now 1.3, so 1.2.13 is relegated to the 'fossils' archive.

The 'fossils' link is described as: "All released versions of zlib" so should work in the future too.

I don't see an official SHA256 for the file on the zlib.net site, so SHA256 calculated as follows:

• download the tar.gz
• verify contents manually
• shasum -a 256