v1.2.12
(21767c654d31d2dccdde4330529775c6c5fd5389)

https://github.com/madler/zlib/releases/tag/v1.2.12

We have also cherry-picked into our local copy:
https://github.com/madler/zlib/commit/ec3df00224d4b396e2ac6586ab5d25f673caa4c2
https://github.com/madler/zlib/commit/7ecf7c7458578d05a20fa481436dd5c58db112f7

And we have inserted into our local copy of zutil.h, line 196, the following declaration:
    ZEXTERN uLong ZEXPORT crc32_combine_gen64 OF((z_off_t));
