Update ZLIB to v1.3.1

[NeroBurner]

ZLIB v1.3.1 got released with bugfixes https://github.com/madler/zlib/releases/tag/v1.3.1

[marc-medley]

As FYI, ZLIB v1.3.1 removes a #define fdopen(fd,mode) NULL statement in zutil.h which breaks ZLIB builds on an up-to-date macOS tool chain.

In particular, v1.3.1 zutil.h has …

#if defined(MACOS)
#  define OS_CODE  7
#endif

… while the previous v1.3.0 zutil.h had …

#if defined(MACOS) || defined(TARGET_OS_MAC)
#  define OS_CODE  7
#  ifndef Z_SOLO
#    if defined(__MWERKS__) && __dest_os != __be_os && __dest_os != __win32_os
#      include <unix.h> /* for fdopen */
#    else
#      ifndef fdopen
#        define fdopen(fd,mode) NULL /* No fdopen() */
#      endif
#    endif
#  endif
#endif

[NeroBurner]

@marc-medley best to report upstream, thanks for the info!