https://bugs.gentoo.org/871177
https://github.com/jedwing/CHMLib/pull/17

From 5877959e3eb9a54e131608c52e2d6f4c89bc0189 Mon Sep 17 00:00:00 2001
From: Florian Weimer <fweimer@redhat.com>
Date: Wed, 19 Apr 2023 09:47:03 +0200
Subject: [PATCH] Avoid implicit function declarations, for C99 compatibility

Define _LARGEFILE64_SOURCE so that <unistd.h> defines pread64.

This avoids build failures with future compilers which do not
support implicit function declarations by default.
--- a/src/chm_lib.c
+++ b/src/chm_lib.c
@@ -48,6 +48,8 @@
  *                                                                         *
  ***************************************************************************/
 
+#define _LARGEFILE64_SOURCE /* for pread64 */
+
 #include "chm_lib.h"
 
 #ifdef CHM_MT