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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<div class='path'>path: <a href='/guix/log/?id=0b95986aaf85c6b81da5b5ddb697fab4d13d308f'>root</a>/<a href='/guix/log/.mailmap?id=0b95986aaf85c6b81da5b5ddb697fab4d13d308f'>.mailmap</a></div><div class='content'><table class='list nowrap'><tr class='nohover'><th class='left'>Age</th><th class='left'>Commit message (<a href='/guix/log/.mailmap?id=0b95986aaf85c6b81da5b5ddb697fab4d13d308f&amp;showmsg=1'>Expand</a>)</th><th class='left'>Author</th></tr>
<tr><td><span title='2019-05-03 08:45:55 +0200'>2019-05-03</span></td><td><a href='/guix/commit/.mailmap?id=3e85909044d8d9b492bcd2ecedde26d71794f145'>Correct email address for ison.</a><span class='msg-avail'>...</span></td><td>ison</td></tr>
<tr><td><span title='2019-03-18 15:05:08 +0100'>2019-03-18</span></td><td><a href='/guix/commit/.mailmap?id=47956fa0c24efcdcba3493f0231c30e9a39ec7df'>Correct name and email address for ng0.</a><span class='msg-avail'>...</span></td><td>ng0</td></tr>
<tr><td><span title='2019-01-21 10:10:43 +0100'>2019-01-21</span></td><td><a href='/guix/commit/.mailmap?id=e3634bd0a3480bde3aba5bd6d84d7aff25413207'>Update .mailmap.</a><span class='msg-avail'>...</span></td><td>Mathieu Othacehe</td></tr>
<tr><td><span title='2018-08-27 14:46:26 +0200'>2018-08-27</span></td><td><a href='/guix/commit/.mailmap?id=4715f92ef0b9650cc4075c69f17aa51e84da3f21'>Update email for Pierre Neidhardt.</a><span class='msg-avail'>...</span></td><td>Pierre Neidhardt</td></tr>
<tr><td><span title='2018-07-16 17:14:39 +0200'>2018-07-16</span></td><td><a href='/guix/commit/.mailmap?id=8bbb79cf95a07a40950448a8a09d888254404ed4'>mailmap: Add a line for Pierre-Antoine.</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2018-07-16 16:53:50 +0200'>2018-07-16</span></td><td><a href='/guix/commit/.mailmap?id=33f4d14ab17096984404fc404b4539d784ecbf32'>mailmap: Add another line for myself.</a><span class='msg-avail'>...</span></td><td>Nils Gillmann</td></tr>
<tr><td><span title='2018-03-20 17:17:58 +0100'>2018-03-20</span></td><td><a href='/guix/commit/.mailmap?id=4a78fd4617908546be15d2c101f5cab512c71f8e'>Correct name and Email for ng0.</a><span class='msg-avail'>...</span></td><td>ng0</td></tr>
<tr><td><span title='2018-02-11 09:44:11 -0500'>2018-02-11</span></td><td><a href='/guix/commit/.mailmap?id=c275462025917ad7b8e182f29d6580804e585643'>mailmap: Adjust entries for ng0.</a><span class='msg-avail'>...</span></td><td>ng0</td></tr>
<tr><td><span title='2017-11-26 18:21:19 -0500'>2017-11-26</span></td><td><a href='/guix/commit/.mailmap?id=9221979e367a95c14a35236742792fb624d8e47c'>Update e-mail for ng0.</a><span class='msg-avail'>...</span></td><td>ng0</td></tr>
<tr><td><span title='2017-10-03 17:07:54 +0200'>2017-10-03</span></td><td><a href='/guix/commit/.mailmap?id=d109b1e85c5c1438e66dfc05ea4a018418edaf19'>Update e-mail for Theodoros Foradis.</a><span class='msg-avail'>...</span></td><td>Theodoros Foradis</td></tr>
<tr><td><span title='2017-08-29 20:13:24 -0400'>2017-08-29</span></td><td><a href='/guix/commit/.mailmap?id=3c8ba11a97d3f3b671b8919da8c41f0b0e2ccaf3'>Update e-mail for Kei Kebreau.</a><span class='msg-avail'>...</span></td><td>Kei Kebreau</td></tr>
<tr><td><span title='2017-08-13 16:11:48 -0400'>2017-08-13</span></td><td><a href='/guix/commit/.mailmap?id=79d5ad20e87066a0937a4b61a4857de2b34a7a30'>mailmap: fix entries for ng0.</a><span class='msg-avail'>...</span></td><td>ng0</td></tr>