The $(top_builddir) is actually not the top_builddir in the build of the package when built in a subdirectory.{}

Executing (target): /var/media/DATA/home-rudi/LibreELEC.tv/build.LibreELEC-Generic.x86_64-11.0-devel/build/zabbix-5.4.9/configure --host=x86_64-libreelec-linux-gnu --build=x86_64-linux-gnu --prefix=/usr --bindir=/usr/bin --sbindir=/usr/sbin --sysconfdir=/etc --libdir=/usr/lib --libexecdir=/usr/lib --localstatedir=/var --disable-static --enable-shared --enable-agent --with-libpcre=/var/media/DATA/home-rudi/LibreELEC.tv/build.LibreELEC-Generic.x86_64-11.0-devel/toolchain

when the build starts the ../../../include is not the directory with the includes. It is actually ../../../../include 

make[1]: Entering directory '/var/media/DATA/home-rudi/LibreELEC.tv/build.LibreELEC-Generic.x86_64-11.0-devel/build/zabbix-5.4.9/.x86_64-libreelec-linux-gnu/src'
Making all in libs
make[2]: Entering directory '/var/media/DATA/home-rudi/LibreELEC.tv/build.LibreELEC-Generic.x86_64-11.0-devel/build/zabbix-5.4.9/.x86_64-libreelec-linux-gnu/src/libs'
Making all in zbxcrypto
make[3]: Entering directory '/var/media/DATA/home-rudi/LibreELEC.tv/build.LibreELEC-Generic.x86_64-11.0-devel/build/zabbix-5.4.9/.x86_64-libreelec-linux-gnu/src/libs/zbxcrypto'
/var/media/DATA/home-rudi/LibreELEC.tv/build.LibreELEC-Generic.x86_64-11.0-devel/toolchain/bin/x86_64-libreelec-linux-gnu-gcc -DHAVE_CONFIG_H -I. -I/var/media/DATA/home-rudi/LibreELEC.tv/build.LibreELEC-Generic.x86_64-11.0-devel/build/zabbix-5.4.9/src/libs/zbxcrypto -I../../../include -I../../../../include     -march=x86-64 -m64 -mmmx -msse -msse2 -mfpmath=sse -Wall -pipe  -O2 -fomit-frame-pointer -DNDEBUG  -I/var/media/DATA/home-rudi/LibreELEC.tv/build.LibreELEC-Generic.x86_64-11.0-devel/toolchain/include -MT libzbxcrypto_a-base64.o -MD -MP -MF .deps/libzbxcrypto_a-base64.Tpo -c -o libzbxcrypto_a-base64.o `test -f 'base64.c' || echo '/var/media/DATA/home-rudi/LibreELEC.tv/build.LibreELEC-Generic.x86_64-11.0-devel/build/zabbix-5.4.9/src/libs/zbxcrypto/'`base64.c

The following is the directory listings. config.h is in the .x86_64-libreelec-linux-gnu/include directory which is included by the source files too.

./zabbix-5.4.9/.x86_64-libreelec-linux-gnu$ ls -la
total 516
drwxr-xr-x  7 docker docker   4096 Jan 24 02:20 .
drwxr-xr-x 14 docker docker   4096 Jan 24 02:20 ..
rw-rr-  1 docker docker 400685 Jan 24 02:20 config.log
-rwxr-xr-x  1 docker docker  56615 Jan 24 02:20 config.status
drwxr-xr-x  6 docker docker   4096 Jan 24 02:20 database
drwxr-xr-x  2 docker docker   4096 Jan 24 02:20 include
rw-rr-  1 docker docker  37130 Jan 24 02:20 Makefile
drwxr-xr-x  2 docker docker   4096 Jan 24 02:20 man
drwxr-xr-x  2 docker docker   4096 Jan 24 02:20 misc
drwxr-xr-x 11 docker docker   4096 Jan 24 02:20 src
./zabbix-5.4.9/.x86_64-libreelec-linux-gnu$ ls -la include/
total 36
drwxr-xr-x 2 docker docker  4096 Jan 24 02:20 .
drwxr-xr-x 7 docker docker  4096 Jan 24 02:20 ..
rw-rr- 1 docker docker 21359 Jan 24 02:20 config.h
rw-rr- 1 docker docker    31 Jan 24 02:20 stamp-h1

The attached patch "works around the issue" and hopefully explains the issue fully.

Package build: will be added to: LibreELEC.tv/packages/addons/service at master - LibreELEC/LibreELEC.tv (github.com)

$ more packages/addons/service/zabbix/package.mk
# SPDX-License-Identifier: GPL-2.0
# Copyright (C) 2022-present Team LibreELEC (https://libreelec.tv)

PKG_NAME="zabbix"
PKG_VERSION="5.4.9"
PKG_SHA256="19686628df76e8d5ef7c2ed2975b258c7ca3ec7d151b1bb59d7e132f9fc7c941"
PKG_REV="100"
PKG_ARCH="any"
PKG_LICENSE="GPL-2.0"
PKG_SITE="http://www.net-snmp.org"
PKG_URL="https://cdn.zabbix.com/${PKG_NAME}/sources/stable/${PKG_VERSION:0:3}/${PKG_NAME}-${PKG_VERSION}.tar.gz"
PKG_DEPENDS_TARGET="toolchain pcre2"
PKG_SECTION="service"
PKG_SHORTDESC="Zabbix is an enterprise-class open source distributed monitoring solution."
PKG_IS_ADDON="yes"
PKG_ADDON_NAME="Zabbix"
PKG_ADDON_TYPE="xbmc.service"PKG_CONFIGURE_OPTS_TARGET="--enable-agent {}
                           --with-libpcre=${TOOLCHAIN}"

addon() {
  mkdir -p ${ADDON_BUILD}/${PKG_ADDON_ID}/lib
  cp -r ${PKG_INSTALL}/storage/.kodi/addons/${PKG_ADDON_ID}/bin ${PKG_INSTALL}/storage/.kodi/userdata/addon_data/${PKG_ADDON_ID}/share ${ADDON_BUILD}/${PKG_ADDON_ID}/
}