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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<div class='path'>path: <a href='/guix/log/?id=3462678bc346c2f6ea81245d6842264b6dccd945'>root</a>/<a href='/guix/log/m4?id=3462678bc346c2f6ea81245d6842264b6dccd945'>m4</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/m4?id=3462678bc346c2f6ea81245d6842264b6dccd945&amp;showmsg=1'>Expand</a>)</th><th class='left'>Author</th></tr>
<tr><td><span title='2020-10-22 17:10:25 +0200'>2020-10-22</span></td><td><a href='/guix/commit/m4?id=59bb1ae3a9aeae75a75b20090253613a7a8800d8'>git: Require Guile-Git 0.3.0 or later.</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2020-09-08 00:47:35 +0200'>2020-09-08</span></td><td><a href='/guix/commit/m4?id=b5eb901ab508e13a10d8c11fe0c9d0d06dddce96'>Remove (guix json) and require Guile-JSON 4.3.0+.</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2020-08-29 16:24:17 +0200'>2020-08-29</span></td><td><a href='/guix/commit/m4?id=f5d3a1690cb78ce9413b78352f1030bf4801617a'>build: Remove check for Guile 2.2.1 bug.</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2020-08-24 13:35:24 +0200'>2020-08-24</span></td><td><a href='/guix/commit/m4?id=4c0c65acfade63ce0549115d19db4b639c1e9992'>Use "guile-zlib" and "guile-lzlib" instead of (guix config).</a><span class='msg-avail'>...</span></td><td>Mathieu Othacehe</td></tr>
<tr><td><span title='2020-07-20 23:25:09 +0200'>2020-07-20</span></td><td><a href='/guix/commit/m4?id=e8088f0b06c2193f2cce04a48aa1350229442a9f'>ssh: Speed up RPCs by using #:nodelay.</a><span class='msg-avail'>...</span></td><td>Lars-Dominik Braun</td></tr>
<tr><td><span title='2020-05-28 11:12:40 +0300'>2020-05-28</span></td><td><a href='/guix/commit/m4?id=03e6404936ef0974fdbc877d537b97b28e54124d'>doc: Remove explicit support for mips64el-linux.</a><span class='msg-avail'>...</span></td><td>Efraim Flashner</td></tr>
<tr><td><span title='2020-05-28 00:55:00 +0200'>2020-05-28</span></td><td><a href='/guix/commit/m4?id=c1dc50ab202fafedde3a6b70ce6339496ad53a51'>maint: Check whether Guile-Gcrypt is recent enough.</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2020-02-22 00:45:58 +0100'>2020-02-22</span></td><td><a href='/guix/commit/m4?id=378918725259d221222dd0b1831fbaa3e0a17e1f'>build: Depend on guile-ssh 0.12.0</a><span class='msg-avail'>...</span></td><td>Lars-Dominik Braun</td></tr>
<tr><td><span title='2019-08-17 19:26:21 +0200'>2019-08-17</span></td><td><a href='/guix/commit/m4?id=611a64bd7e0893057cd04dcdc2a8d5e7ecc39e45'>build: 'GUIX_CHECK_GUILE_JSON' really checks for Guile-JSON 3.x.</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2019-07-25 00:16:41 +0200'>2019-07-25</span></td><td><a href='/guix/commit/m4?id=81c3dc32244a17241d74eea9fa265edfcb326f6d'>maint: Switch to Guile-JSON 3.x.</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2019-05-06 23:21:33 +0200'>2019-05-06</span></td><td><a href='/guix/commit/m4?id=fea338c6ca1922097fa233be85f424c152a4f507'>Add (guix lzlib).</a><span class='msg-avail'>...</span></td><td>Pierre Neidhardt</td></tr>
<tr><td><span title='2018-11-23 15:42:00 +0100'>2018-11-23</span></td><td><a href='/guix/commit/m4?id=60e1c1099fc3d73ed7d3235e71aae5d00ab7d773'>Update Guile-SQLite3 URL everywhere.</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2018-09-04 17:25:11 +0200'>2018-09-04</span></td><td><a href='/guix/commit/m4?id=ca719424455465fca4b872c371daf2a46de88b33'>Switch to Guile-Gcrypt.</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2018-06-01 15:35:15 +0200'>2018-06-01</span></td><td><a href='/guix/commit/m4?id=d59e75f3b5b82692bd250a1a3a9965397bb588c5'>build: Check for Guile-SQLite3.</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>