Fix `ssize_t` for mingw; ref
Legacy MinGW and MinGW-w64 had own `ssize_t`.
typedef long fsuseconds_t;
typedef int fmode_t;
typedef int mrb_io_read_write_size;
+ #if !defined(_SSIZE_T_) && !defined(_SSIZE_T_DEFINED) && \
+ !defined(__have_typedef_ssize_t)
typedef SSIZE_T ssize_t;
+ #endif
#ifndef O_TMPFILE
#define O_TMPFILE O_TEMPORARY
