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='2021-05-20 01:58:04 +0200'>2021-05-20</span></td><td><a href='/guix/commit/ROADMAP?id=c0087d33705f53cc3619a1a43a33602aaecc3a8b'>The #guix channel is hosted by Libera Chat.</a><span class='msg-avail'>...</span></td><td>Tobias Geerinckx-Rice</td></tr>