Detail:
  zlib 1.2.12 update:
  - README, c/*, h/*, doc/* - Updated to zlib 1.2.12
  Changes compared to the official zlib distribution:
  - h/zutil - Use OS_CODE of 0x0d for RISC OS, as decreed by rfc1952
  - c/gzlib, c/gzread, c/gzwrite, h/zconf, h/zlib, h/gzguts - Reintroduced ability to use ISO file access functions instead of POSIX ones, to allow the library to be easily used with Norcroft. Added gzgetheader() and gzputheader() functions to allow easy read/write of gzip file headers via the gzip file interface