Condition roken's ssize_t definition for compatibility

If SSIZE_T_DEFINED macro is defined, assume we already have ssize_t.

lib/roken/roken.h.in

@@ -262,12 +262,18 @@ struct sockaddr_dl;
 #endif
 
 #ifndef HAVE_SSIZE_T
+#ifndef SSIZE_T_DEFINED
+#ifdef ssize_t
+#undef ssize_t
+#endif
 #ifdef _WIN64
 typedef __int64 ssize_t;
 #else
 typedef int ssize_t;
 #endif
-#endif
+#define SSIZE_T_DEFINED
+#endif  /* SSIZE_T_DEFINED */
+#endif  /* HAVE_SSIZE_T */
 
 #include <roken-common.h>