portability updates

add check if ipv6/sockaddr_in6 is availavble or not
add check for poll.h and only include when available
add includes for AROS

lib/md5.c

@@ -19,6 +19,13 @@
  *  - Ian Jackson <ijackson@nyx.cs.du.edu>.
  * Still in the public domain.
  */
+#ifdef HAVE_CONFIG_H
+#include "config.h"
+#endif
+
+#ifdef HAVE_SYS_TYPES_H
+#include <sys/types.h>
+#endif
 
 #include "md5.h"