build BUGFIX fix BSD builds, again, and cleanup

- just define _GNU_SOURCE and include sys/cdefs.h this gets everything
set the right way (no need to define _POSIX_C_SOURCE, etc).
- BSD requires more includes to get to sockaddr and AF_*
diff --git a/src/context.c b/src/context.c
index 9ae7213..b5fe9b4 100644
--- a/src/context.c
+++ b/src/context.c
@@ -11,14 +11,13 @@
  *
  *     https://opensource.org/licenses/BSD-3-Clause
  */
-#define _GNU_SOURCE /* asprintf */
-#define _POSIX_C_SOURCE 200809L /* strdup */
-
+#define _GNU_SOURCE /* asprintf, strdup */
 #if defined (__NetBSD__) || defined (__OpenBSD__)
 /* realpath */
 #define _XOPEN_SOURCE 1
 #define _XOPEN_SOURCE_EXTENDED 1
 #endif
+#include <sys/cdefs.h>
 
 #include "context.h"