https://github.com/robbat2/readahead-list/pull/1

From 37e699689661249c88a020681a70cd0633ee7ef4 Mon Sep 17 00:00:00 2001
From: Sam James <sam@gentoo.org>
Date: Sun, 25 Dec 2022 20:34:38 +0000
Subject: [PATCH 2/6] Set -D_GNU_SOURCE for readahead() to fix
 -Wimplicit-function-declaration
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readahead-list.c: In function ‘process_file’:
readahead-list.c:111:17: error: implicit declaration of function ‘readahead’ [-Werror=implicit-function-declaration]
  111 |                 readahead(fd, (loff_t)0, (size_t)buf.st_size);
      |                 ^~~~~~~~~

Signed-off-by: Sam James <sam@gentoo.org>
--- a/configure.ac
+++ b/configure.ac
@@ -40,6 +40,9 @@ AC_PATH_PROG(CMP, cmp, cmp)
 AC_CANONICAL_HOST
 AC_PROG_RANLIB
 
+# Provides _GNU_SOURCE for readahead()
+AC_USE_SYSTEM_EXTENSIONS
+
 # Checks for header files.
 AC_CHECK_HEADERS([ errno.h fcntl.h getopt.h linux/fs.h sched.h stdio.h stdlib.h string.h sys/ioctl.h sys/mman.h sys/syscall.h sys/types.h unistd.h ])
 
--- a/src/filelist-order.cxx
+++ b/src/filelist-order.cxx
@@ -14,6 +14,9 @@
 // along with this program; if not, write to the Free Software 
 // Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA.
 //
+
+#include <../config.h>
+
 #include <map>
 #include <set>
 #include <iostream>
@@ -31,7 +34,6 @@
 
 #include <fcntl.h>
 #include <linux/fs.h>
-#include <../config.h>
 
 #ifndef BUFFER_SIZE
 #define BUFFER_SIZE 2048
--- a/src/readahead-list.c
+++ b/src/readahead-list.c
@@ -19,6 +19,7 @@
 // Based on code originally written by Erich Schubert <erich@debian.org>.
 // $Header: /code/convert/cvsroot/infrastructure/readahead-list/src/readahead-list.c,v 1.5 2006/04/21 10:13:54 robbat2 Exp $
 
+#include <../config.h>
 #include <stdio.h>
 #include <stdlib.h>
 #include <unistd.h>
@@ -31,7 +32,6 @@
 #include <sched.h>
 #include <string.h>
 #include <errno.h>
-#include <../config.h>
 
 #if __STDC_VERSION__ < 199901L
 # if __GNUC__ >= 2
-- 
2.39.0