v1.3.1
(51b7f2abdade71cd9bb0e7a373ef2610ec6f9daf)

https://github.com/madler/zlib/releases/tag/v1.3.1

We have removed zlib.3.pdf from our local copy, as it is a binary file which is
not needed for our compilation.

We have also applied the custom patches under the patches/zlib folder.
