_POSIX_C_SOURCE needs to be set to a date, at least on FreeBSD.

Change-Id: I4020bb6ecd3d7ed745cdba89e50536eafab1f1ee
Reviewed-on: https://boringssl-review.googlesource.com/1851
Reviewed-by: Adam Langley <agl@google.com>

crypto/directory_posix.c

@@ -25,7 +25,7 @@
  * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE. */
 
 #if !defined(_POSIX_C_SOURCE)
-#define _POSIX_C_SOURCE 1  /* for readdir_r */
+#define _POSIX_C_SOURCE 201409  /* for readdir_r */
 #endif
 
 #include "directory.h"