This patch fix error on 'gnulib' library required to build
'grep' package on GNU/Hurd.
The patch was adapted from upstream source repository:
'<http://git.savannah.gnu.org/gitweb/?p=gnulib.git;a=commit;h=4084b3a1094372b960ce4a97634e08f4538c8bdd>'

Commit: 4084b3a1094372b960ce4a97634e08f4538c8bdd

diff --git a/lib/glthread/lock.c b/lib/glthread/lock.c
index 061562b..afc86f4 100644
--- a/lib/glthread/lock.c
+++ b/lib/glthread/lock.c
@@ -30,7 +30,7 @@
 
 /* ------------------------- gl_rwlock_t datatype ------------------------- */
 
-# if HAVE_PTHREAD_RWLOCK && (HAVE_PTHREAD_RWLOCK_RDLOCK_PREFER_WRITER || (__GNU_LIBRARY__ > 1))
+# if HAVE_PTHREAD_RWLOCK && (HAVE_PTHREAD_RWLOCK_RDLOCK_PREFER_WRITER || (defined PTHREAD_RWLOCK_WRITER_NONRECURSIVE_INITIALIZER_NP && (__GNU_LIBRARY__ > 1)))
 
 #  ifdef PTHREAD_RWLOCK_INITIALIZER
 
diff --git a/lib/glthread/lock.h b/lib/glthread/lock.h
index ec16d39..67932aa 100644
--- a/lib/glthread/lock.h
+++ b/lib/glthread/lock.h
@@ -179,7 +179,7 @@ typedef pthread_mutex_t gl_lock_t;
 
 /* ------------------------- gl_rwlock_t datatype ------------------------- */
 
-# if HAVE_PTHREAD_RWLOCK && (HAVE_PTHREAD_RWLOCK_RDLOCK_PREFER_WRITER || (__GNU_LIBRARY__ > 1))
+# if HAVE_PTHREAD_RWLOCK && (HAVE_PTHREAD_RWLOCK_RDLOCK_PREFER_WRITER || (defined PTHREAD_RWLOCK_WRITER_NONRECURSIVE_INITIALIZER_NP && (__GNU_LIBRARY__ > 1)))
 
 #  ifdef PTHREAD_RWLOCK_INITIALIZER
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<div class='path'>path: <a href='/guix/log/?id=e101a117eb5af9affcf68e27e91d68d3bfd72986'>root</a>/<a href='/guix/log/etc?id=e101a117eb5af9affcf68e27e91d68d3bfd72986'>etc</a>/<a href='/guix/log/etc/system-tests.scm?id=e101a117eb5af9affcf68e27e91d68d3bfd72986'>system-tests.scm</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/etc/system-tests.scm?id=e101a117eb5af9affcf68e27e91d68d3bfd72986&amp;showmsg=1'>Expand</a>)</th><th class='left'>Author</th></tr>
<tr><td><span title='2020-03-23 00:06:58 +0100'>2020-03-23</span></td><td><a href='/guix/commit/etc/system-tests.scm?id=8b18f5cb309f670b08ad35df6425f4e9e19d8a82'>tests: Use the same source as ci.guix.gnu.org.</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2020-03-05 16:33:38 +0100'>2020-03-05</span></td><td><a href='/guix/commit/etc/system-tests.scm?id=5ec4156bbcaec8337f78411204d59e59e706103b'>tests: Add a manifest for system tests.</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>