make sure _XOPEN_SOURCE gets defined before posix_memalign is used

src/include/field.h

@@ -9,8 +9,8 @@
 #ifndef __FIELD_H__
 #define __FIELD_H__
 
-#include <string.h>
 #include "constant_time.h"
+#include <string.h>
 
 #include "p448.h"
 #define FIELD_BITS           448

src/include/word.h

@@ -5,11 +5,12 @@
 #ifndef __WORD_H__
 #define __WORD_H__
 
-#include "arch_config.h"
-
 /* for posix_memalign */
 #define _XOPEN_SOURCE 600
 
+#include "arch_config.h"
+
+
 #ifndef __APPLE__
 #ifndef _BSD_SOURCE
 #define _BSD_SOURCE 1