This patch removes the _XOPEN_SOURCE preprocessor directive as it does not seem to be needed.
Setting it removes the definition of strdup, which is used in filter-datatypes.c.

Patch by Roel Janssen <roel@gnu.org>
*** a/src/frontend/filter-datatypes.c	1970-01-01 01:00:00.000000000 +0100
--- b/src/frontend/filter-datatypes.c	2018-04-03 17:39:23.177905592 +0200
***************
*** 18,24 ****
   *  Copyright (C) 2006 Steve Harris for Garlik
   */
  
- #define _XOPEN_SOURCE
  #include <stdlib.h>
  #include <string.h>
  #include <math.h>
--- 18,23 ----
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