Skip "test-rwlock1" when multithreading is disabled, which is the case
during bootstrapping on architectures not supported by GNU Mes.

Taken from upstream gnulib:
https://git.savannah.gnu.org/cgit/gnulib.git/commit/?id=fdff8bd09a7f053381f8bdb107ab5280b7c95959

diff --git a/gnulib-tests/test-rwlock1.c b/gnulib-tests/test-rwlock1.c
--- a/gnulib-tests/test-rwlock1.c
+++ b/gnulib-tests/test-rwlock1.c
@@ -21,6 +21,8 @@
 
 #include <config.h>
 
+#if USE_ISOC_THREADS || USE_POSIX_THREADS || USE_ISOC_AND_POSIX_THREADS || USE_WINDOWS_THREADS
+
 #include "glthread/lock.h"
 
 #include <errno.h>
@@ -151,3 +153,18 @@ main ()
       sleep (1);
     }
 }
+
+#else
+
+/* No multithreading available.  */
+
+#include <stdio.h>
+
+int
+main ()
+{
+  fputs ("Skipping test: multithreading not enabled\n", stderr);
+  return 77;
+}
+
+#endif

ts</option>
</select> <input type='submit' value='switch'/></form></td></tr>
<tr><td class='sub'>Wojtek's customized Guix</td><td class='sub right'></td></tr></table>
<table class='tabs'><tr><td>
<a href='/guix/about/'>about</a><a href='/guix/'>summary</a><a href='/guix/refs/?id=897186c1554e18a187429683d2eafdbfc0caac96'>refs</a><a class='active' href='/guix/log/gnu/packages/inklingreader.scm'>log</a><a href='/guix/tree/gnu/packages/inklingreader.scm?id=897186c1554e18a187429683d2eafdbfc0caac96'>tree</a><a href='/guix/commit/gnu/packages/inklingreader.scm?id=897186c1554e18a187429683d2eafdbfc0caac96'>commit</a><a href='/guix/diff/gnu/packages/inklingreader.scm?id=897186c1554e18a187429683d2eafdbfc0caac96'>diff</a></td><td class='form'><form class='right' method='get' action='/guix/log/gnu/packages/inklingreader.scm'>
<input type='hidden' name='id' value='897186c1554e18a187429683d2eafdbfc0caac96'/><select name='qt'>
<option value='grep'>log msg</option>
<option value='author'>author</option>
<option value='committer'>committer</option>
<option value='range'>range</option>
</select>
<input class='txt' type='search' size='10' name='q' value=''/>
<input type='submit' value='search'/>
</form>
</td></tr></table>
<div class='path'>path: <a href='/guix/log/?id=897186c1554e18a187429683d2eafdbfc0caac96'>root</a>/<a href='/guix/log/gnu?id=897186c1554e18a187429683d2eafdbfc0caac96'>gnu</a>/<a href='/guix/log/gnu/packages?id=897186c1554e18a187429683d2eafdbfc0caac96'>packages</a>/<a href='/guix/log/gnu/packages/inklingreader.scm?id=897186c1554e18a187429683d2eafdbfc0caac96'>inklingreader.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/gnu/packages/inklingreader.scm?id=897186c1554e18a187429683d2eafdbfc0caac96&amp;showmsg=1'>Expand</a>)</th><th class='left'>Author</th></tr>