what is _XOPEN_SOURCE=500 needed for?

[Matthieu Herrb]

Hi,

I'm trying to build the modular tree on OpenBSD. One problem I noticed 
is that -D_XOPEN_SOURCE=500 is now added unconditionnaly to all CFLAGS.
This breaks things on OpenBSD, so It should probably be defined only on 
systems that needs it.
One problem when dealing with this kind of macros and autoconf is to 
find out what features exactly need this define.
So does any one know why it's needed on Linux, Solaris and cygwin (these 
are the only platforms that define it in the monolithic tree).
-- 
Matthieu Herrb