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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<tr><td><span title='2019-01-25 13:21:19 +0100'>2019-01-25</span></td><td><a href='/guix/commit/gnu/packages/textutils.scm?id=0ad0eceed4ffdf1c5da8737edc15f429ac077775'>gnu: antiword: Use INVOKE.</a><span class='msg-avail'>...</span></td><td>Ricardo Wurmus</td></tr>
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<tr><td><span title='2018-12-23 19:48:21 +0200'>2018-12-23</span></td><td><a href='/guix/commit/gnu/packages/textutils.scm?id=bf5af9342c6dd364242c162db00436c9708e8d5b'>gnu: opencc: Use 'git-fetch'.</a><span class='msg-avail'>...</span></td><td>Efraim Flashner</td></tr>
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<tr><td><span title='2018-11-11 11:36:39 +0100'>2018-11-11</span></td><td><a href='/guix/commit/gnu/packages/textutils.scm?id=7a52d0e075baebe2c04331b82d5974f2db9bf9f0'>gnu: dotconf: Remove custom bootstrap phase.</a><span class='msg-avail'>...</span></td><td>Ricardo Wurmus</td></tr>
<tr><td><span title='2018-11-11 11:34:55 +0100'>2018-11-11</span></td><td><a href='/guix/commit/gnu/packages/textutils.scm?id=d68a15806b26ee23bf83c92ed29d2d4b1024b919'>gnu: libgtextutils: Remove custom bootstrap phase.</a><span class='msg-avail'>...</span></td><td>Ricardo Wurmus</td></tr>
<tr><td><span title='2018-11-10 23:33:46 +0100'>2018-11-10</span></td><td><a href='/guix/commit/gnu/packages/textutils.scm?id=1a0363cf0794bb7f8149800b665f80728b3635ff'>gnu: Add opencc.</a><span class='msg-avail'>...</span></td><td>Meiyo Peng</td></tr>
<tr><td><span title='2018-11-01 04:34:24 +0100'>2018-11-01</span></td><td><a href='/guix/commit/gnu/packages/textutils.scm?id=351a12b0a7e57c9aa95e2780e7e568597a425892'>gnu: oniguruma: Update to 6.9.0.</a><span class='msg-avail'>...</span></td><td>Tobias Geerinckx-Rice</td></tr>
<tr><td><span title='2018-10-09 05:42:14 +0200'>2018-10-09</span></td><td><a href='/guix/commit/gnu/packages/textutils.scm?id=f6ccb95a86ef22ae678e80b0e9a353e5b2404334'>gnu: docx2txt: End phases with #t.</a><span class='msg-avail'>...</span></td><td>Tobias Geerinckx-Rice</td></tr>
<tr><td><span title='2018-10-01 23:02:40 +0200'>2018-10-01</span></td><td><a href='/guix/commit/gnu/packages/textutils.scm?id=08cbd4f3b9ce0bc6aa052b22795a3a9b9f09b04c'>gnu: recode: Update to 3.7.1.</a><span class='msg-avail'>...</span></td><td>Tobias Geerinckx-Rice</td></tr>
<tr><td><span title='2018-08-27 14:46:26 +0200'>2018-08-27</span></td><td><a href='/guix/commit/gnu/packages/textutils.scm?id=4715f92ef0b9650cc4075c69f17aa51e84da3f21'>Update email for Pierre Neidhardt.</a><span class='msg-avail'>...</span></td><td>Pierre Neidhardt</td></tr>
<tr><td><span title='2018-07-12 17:21:17 +0200'>2018-07-12</span></td><td><a href='/guix/commit/gnu/packages/textutils.scm?id=d5019f9ca750aa2bd49048c5462d9eac220b0644'>gnu: php: Build with an older oniguruma.</a><span class='msg-avail'>...</span></td><td>Marius Bakke</td></tr>
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<tr><td><span title='2018-07-07 18:10:32 +0200'>2018-07-07</span></td><td><a href='/guix/commit/gnu/packages/textutils.scm?id=74fa77e936d88a82fbb6b625beae49287b8653e9'>gnu: Add docx2txt.</a><span class='msg-avail'>...</span></td><td>Pierre Neidhardt</td></tr>