[msvc] Define SSIZE_MAX if not already defined (i.e. on Windows)

limits.h from MSVC does not include SIZE_MAX or SSIZE_MAX. We need the latter to build sgen, so define it.
Merge pull request #1420 from directhex/define_ssize_max_on_windows

[msvc] Define SSIZE_MAX if not already defined (i.e. on Windows)