eclass/out-of-source-utils: Add from Gentoo

It's from Gentoo commit 579b4fc3cc9e0f055eb5cc257b822358dddf7fb8.

sdk_container/src/third_party/portage-stable/eclass/out-of-source-utils.eclass

@@ -0,0 +1,43 @@
+# Copyright 2022-2023 Gentoo Authors
+# Distributed under the terms of the GNU General Public License v2
+
+# @ECLASS: out-of-source-utils.eclass
+# @MAINTAINER:
+# Michał Górny <mgorny@gentoo.org>
+# @AUTHOR:
+# Michał Górny <mgorny@gentoo.org>
+# @SUPPORTED_EAPIS: 6 7 8
+# @BLURB: Utility functions for building packages out-of-source
+# @DESCRIPTION:
+# This eclass provides a run_in_build_dir() helper that can be used
+# to execute specified command inside BUILD_DIR.
+
+case ${EAPI} in
+	6|7|8) ;;
+	*) die "${ECLASS}: EAPI ${EAPI:-0} not supported" ;;
+esac
+
+if [[ ! ${_OUT_OF_SOURCE_UTILS_ECLASS} ]]; then
+_OUT_OF_SOURCE_UTILS_ECLASS=1
+
+# @FUNCTION: run_in_build_dir
+# @USAGE: <argv>...
+# @DESCRIPTION:
+# Run the given command in the directory pointed by BUILD_DIR.
+run_in_build_dir() {
+	debug-print-function ${FUNCNAME} "${@}"
+	local ret
+
+	[[ ${#} -eq 0 ]] && die "${FUNCNAME}: no command specified."
+	[[ -z ${BUILD_DIR} ]] && die "${FUNCNAME}: BUILD_DIR not set."
+
+	mkdir -p "${BUILD_DIR}" || die
+	pushd "${BUILD_DIR}" >/dev/null || die
+	"${@}"
+	ret=${?}
+	popd >/dev/null || die
+
+	return ${ret}
+}
+
+fi