windows: add SSIZE_MAX

Windows doesn't include ssize_t or its _MAX value by default.  We are
already declaring ssize_t as SSIZE_T, which is __int64_t on Win64 and
long otherwise.  Include its _MAX value as a correspondence to its type.

src/win32/msvc-compat.h

@@ -12,6 +12,12 @@
 typedef unsigned short mode_t;
 typedef SSIZE_T ssize_t;
 
+#ifdef _WIN64
+# define SSIZE_MAX _I64_MAX
+#else
+# define SSIZE_MAX LONG_MAX
+#endif
+
 #define strcasecmp(s1, s2) _stricmp(s1, s2)
 #define strncasecmp(s1, s2, c) _strnicmp(s1, s2, c)