Fix build error and refine definition of `ssize_t` on MSVC.

include/mruby/common.h

@@ -28,6 +28,11 @@
 # define MRB_END_DECL
 #endif
 
+#if defined _MSC_VER
+#include <BaseTsd.h>
+typedef SSIZE_T ssize_t;
+#endif
+
 /**
  * Shared compiler macros
  */

mrbgems/mruby-io/src/io.c

@@ -34,12 +34,6 @@
   typedef long fsuseconds_t;
   typedef int fmode_t;
   typedef int mrb_io_read_write_size;
-  #ifndef MRB_MINGW32_LEGACY
-    #if !defined(_SSIZE_T_) && !defined(_SSIZE_T_DEFINED) && \
-        !defined(__have_typedef_ssize_t)
-    typedef SSIZE_T ssize_t;
-    #endif
-  #endif
 
   #ifndef O_TMPFILE
     #define O_TMPFILE O_TEMPORARY

mrbgems/mruby-socket/src/socket.c

@@ -13,9 +13,6 @@
   #include <winerror.h>
 
   #define SHUT_RDWR SD_BOTH
-  #ifndef _SSIZE_T_DEFINED
-  typedef int ssize_t;
-  #endif
   typedef int fsize_t;
 #else
   #include <sys/types.h>