fixed ssize_t typedef; added a define to detect already defined ssize_t.

include/libssh2.h

@@ -72,8 +72,11 @@ typedef unsigned int uint32_t;
 #if defined(LIBSSH2_WIN32) && defined(_MSC_VER) && (_MSC_VER <= 1400)
 typedef unsigned __int64 libssh2_uint64_t;
 typedef __int64 libssh2_int64_t;
-typedef long ssize_t;
 typedef unsigned int uint32_t;
+#ifndef _SSIZE_T_DEFINED
+typedef int ssize_t;
+#define _SSIZE_T_DEFINED
+#endif
 #else
 typedef unsigned long long libssh2_uint64_t;
 typedef long long libssh2_int64_t;