Add _XOPEN_SOURCE_EXTENDED define to allow compiling on OpenBSD.

[fcambus]

OpenBSD also needs the _XOPEN_SOURCE_EXTENDED define.

[edgar-bonet]

What about NetBSD and DragonFly BSD? Would it make sense to use a test that matches all the BSDs?

[fcambus]

I can't test on NetBSD or DragonFlyBSD, but I think testing for the BSD macro being defined isn't a good idea, in practice this idiom is rarely used in the wild.

I would suggest to keep this OpenBSD specific for now.