By default, glibc 2.8 hides struct ucred behind the _GNU_SOURCE
macro. I don't want to enable that globally, because it may encourage
the use of non-portable functions. Test if "struct ucred" is
available, and enable _GNU_SOURCE if required.
For details about that issue, see glib's bug database:
http://sources.redhat.com/bugzilla/show_bug.cgi?id=6545
Max Kellermann