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Looking at it again, undefing _XOPEN_SOURCE is not a good
idea.
Yes you'll miss the data types in socketmodule.c too but
they will be addressed 
in another patch to Include/pyport.h which is not included
in the configure tests.

I haven't taken the time to analize why it works (I didn't
write it). It's been
working fine on a wide range of platforms for over a year in
OpenSSH portable.
I wanted to get it in 2.2.2 but it requires autoconf 2.52 or
newer.
I've personaly tested on Linux, Solaris, UnixWare,
OpenServer.
UnixWare uses size_t and OpenServer uses int.

My new patch gets rid of AC_DEFUN and adds some data type
tests.
Two of the data type tests (u_char & u_long) are not needed
by the socklen_t
test but will be needed by a UnixWare 7 patch that I'll post
shortly.

Expect some fuz in the patch.  I'm breaking up a large patch
into understanable
chunks.