zlib 1.3.1.
Files not required by OpenMPT have been removed.
OpenMPT specific changes:
 *  iowin32.c has been modified to only use Windows 8 WinRT API when actually
    compiling for Windows 8 or later.
 *  minizip has been modified to check for allocation failure. A similar fix
    has been proposed upstream: <https://github.com/madler/zlib/pull/566>
 *  All custom modifications are marked with /* OpenMPT */
 *  For building, premake is used to generate Visual Studio project files.
    See ../build/premake/ for details.
No further changes have been made.
