configure.ac: check if "struct ucred" is available

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

src/listen.c

@@ -97,7 +97,7 @@ static int establishListen(int pf, const struct sockaddr *addrp,
 	if (listen(sock, 5) < 0)
 		FATAL("problems listen'ing: %s\n", strerror(errno));
 
-#if defined(HAVE_UN) && defined(SO_PASSCRED)
+#ifdef HAVE_UCRED
 	setsockopt(sock, SOL_SOCKET, SO_PASSCRED, &passcred, sizeof(passcred));
 #endif
 
@@ -265,7 +265,7 @@ void freeAllListenSockets(void)
 
 static int get_remote_uid(int fd)
 {
-#if defined(HAVE_UN) && defined(SO_PEERCRED)
+#ifdef HAVE_UCRED
 	struct ucred cred;
 	socklen_t len = sizeof (cred);