In zlib 1.2.12 the OF macro was changed to _Z_OF breaking any
project that used zlibWrapper. To fix this the OF has been
changed to _Z_OF everywhere and _Z_OF is defined as OF in the
case it is not yet defined for zlib 1.2.11 and older.
Fixes: https://github.com/facebook/zstd/issues/3216
As per https://github.com/facebook/zstd/issues/1800#issuecomment-545945050,
Visual does not support `ssize_t` type,
which is an issue for `gzread.c`.
Added a work around, suggested by @bluenlive
Note : I have not been able to confirm the problem,
so this is a blind fix.
This seems safe outside of Visual, since it is gated by _MSC_VER macro.
