Don't redefine 'closefrom' if available through glibc.

Shamelessly stolen from Gentoo:

  https://gitweb.gentoo.org/repo/gentoo.git/tree/sys-fs/fuse/files/fuse-2.9.9-closefrom-glibc-2-34.patch

diff --git a/configure.ac b/configure.ac
index 9946a0efa..a2d481aa9 100644
--- a/configure.ac
+++ b/configure.ac
@@ -55,6 +55,7 @@ fi
 
 AC_CHECK_FUNCS([fork setxattr fdatasync splice vmsplice utimensat])
 AC_CHECK_FUNCS([posix_fallocate])
+AC_CHECK_FUNCS([closefrom])
 AC_CHECK_MEMBERS([struct stat.st_atim])
 AC_CHECK_MEMBERS([struct stat.st_atimespec])
 
diff --git a/util/ulockmgr_server.c b/util/ulockmgr_server.c
index 273c7d923..a04dac5c6 100644
--- a/util/ulockmgr_server.c
+++ b/util/ulockmgr_server.c
@@ -22,6 +22,10 @@
 #include <sys/socket.h>
 #include <sys/wait.h>
 
+#ifdef HAVE_CONFIG_H
+	#include "config.h"
+#endif
+
 struct message {
 	unsigned intr : 1;
 	unsigned nofd : 1;
@@ -124,6 +128,7 @@ static int receive_message(int sock, void *buf, size_t buflen, int *fdp,
 	return res;
 }
 
+#if !defined(HAVE_CLOSEFROM)
 static int closefrom(int minfd)
 {
 	DIR *dir = opendir("/proc/self/fd");
@@ -141,6 +146,7 @@ static int closefrom(int minfd)
 	}
 	return 0;
 }
+#endif
 
 static void send_reply(int cfd, struct message *msg)
 {
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<tr><td><span title='2022-04-29 18:07:17 +0200'>2022-04-29</span></td><td><a href='/guix/commit/gnu/tests/version-control.scm?id=73eeeeafbb0765f76834b53c9fe6cf3c8f740840'>tests: Avoid starting services to check whether they're running.</a><span class='msg-avail'>...<span class='msg-tooltip'>Fixes a regression introduced in the Shepherd 0.9.0 whereby clients can
start a service that is already being started, leading to two instances
of the process (and usually failure of the second attempt to start it).

Partly fixes &lt;https://issues.guix.gnu.org/54786&gt;.
Reported by Mathieu Othacehe.

* gnu/tests/version-control.scm (run-cgit-test)
["nginx running", "fcgiwrap running"]: Wail for a file/TCP port instead
of starting the service.
(run-git-http-test)["nginx running"]: Likewise.
(run-gitile-test)["nginx running"]: Likewise.
</span></span></td><td>Ludovic Courtès</td></tr>