+#if _POSIX_C_SOURCE >= 200809L || _ATFILE_SOUCE
+# define EXIM_HAVE_OPENAT
+#endif
+
+#include <netinet/tcp.h> /* for TCP_FASTOPEN */
+#include <sys/socket.h> /* for MSG_FASTOPEN */
+#if defined(TCP_FASTOPEN) && !defined(MSG_FASTOPEN)
+# define MSG_FASTOPEN 0x20000000
+#endif
+