3.2.2p1 build problem on Irix6.5

[Dave Love]

On Irix 6.5.15m with version 7.3 of the development tools,
monitor_fdpass.c won't compile after a normal configure because
SCM_RIGHTS isn't defined.  In sys/socket.h it's protected by
#ifdef _XOPEN_SOURCE but if I define _XOPEN_SOURCE, I get a storm of
IPV6-related errors.  Defining SCM_RIGHTS explicitly does allow it to
build.  I can't immediately figure out how to fix this properly.