BUILD: adjust makefile for AIX 5.1

AIX 5.1 has trouble with ss_family which is __ss_family there.
Just remap it in the makefile and provide a new target.
diff --git a/README b/README
index 0f3a1f9..d2413a8 100644
--- a/README
+++ b/README
@@ -30,6 +30,7 @@
   - freebsd     for FreeBSD 5 to 8.0 (others untested)
   - osx         for Mac OS/X
   - openbsd     for OpenBSD 3.1 to 5.2 (others untested)
+  - aix51       for AIX 5.1
   - aix52       for AIX 5.2
   - cygwin      for Cygwin
   - generic     for any other OS.