prlimit(2) returns ENOSYS on Linux 2.6.32-5-xen-amd64 as found on
hydra.gnu.org, and strace(1) doesn't recognize it.

--- glib-2.34.3/glib/tests/thread.c	2012-11-20 15:27:12.000000000 +0100
+++ glib-2.34.3/glib/tests/thread.c	2013-03-27 14:48:31.000000000 +0100
@@ -130,7 +130,7 @@ test_thread3 (void)
 static void
 test_thread4 (void)
 {
-#ifdef HAVE_PRLIMIT
+#if 0
   struct rlimit ol, nl;
   GThread *thread;
   GError *error;
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<tr><td><span title='2020-12-08 22:30:07 +0100'>2020-12-08</span></td><td><a href='/guix/commit/nix/libutil/util.hh?id=2e308238ad09e9527f9b8c3ec4a90b6b05a93367'>daemon: 'Agent' constructor takes a list of environment variables.</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2020-09-14 15:42:55 +0200'>2020-09-14</span></td><td><a href='/guix/commit/nix/libutil/util.hh?id=ee9dff34f9317509cb2b833d07a0d5e01a36a4ae'>daemon: Move 'Agent' to libutil.</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2020-01-12 00:19:09 +0100'>2020-01-12</span></td><td><a href='/guix/commit/nix/libutil/util.hh?id=7033c7692ccbbbad8f7b9952015de071a5588e87'>daemon: Account for deleted store files when deduplication is on.</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2017-08-18 11:31:28 +0200'>2017-08-18</span></td><td><a href='/guix/commit/nix/libutil/util.hh?id=ed464f48efd80e6439441d7a9f6d0e7988fb1272'>nix: Remove unused function.</a><span class='msg-avail'>...</span></td><td>Andy Wingo</td></tr>