eclass/app-alternatives: Add from Gentoo

It's from Gentoo commit b0c939c07feb8d0158925dce14735a5c8439d7e4.

sdk_container/src/third_party/portage-stable/eclass/app-alternatives.eclass

@@ -0,0 +1,84 @@
+# Copyright 2022 Gentoo Authors
+# Distributed under the terms of the GNU General Public License v2
+
+# @ECLASS: app-alternatives.eclass
+# @MAINTAINER:
+# Michał Górny <mgorny@gentoo.org>
+# @AUTHOR:
+# Michał Górny <mgorny@gentoo.org>
+# @SUPPORTED_EAPIS: 8
+# @BLURB: Common logic for app-alternatives/*
+# @DESCRIPTION:
+# This eclass provides common logic shared by app-alternatives/*
+# ebuilds.  A global ALTERNATIVES variable needs to be declared
+# that lists available options and their respective dependencies.
+# HOMEPAGE, S, LICENSE, SLOT, IUSE, REQUIRED_USE and RDEPEND are set.
+# A get_alternative() function is provided that determines the selected
+# alternative and prints its respective flag name.
+
+case ${EAPI} in
+	8) ;;
+	*) die "${ECLASS}: EAPI ${EAPI:-0} unsupported."
+esac
+
+if [[ ! ${_APP_ALTERNATIVES_ECLASS} ]]; then
+_APP_ALTERNATIVES_ECLASS=1
+
+# @ECLASS_VARIABLE: ALTERNATIVES
+# @PRE_INHERIT
+# @REQUIRED
+# @DESCRIPTION:
+# Array of "flag:dependency" pairs specifying the available
+# alternatives.  The default provider must be listed first.
+
+# @FUNCTION: _app-alternatives_set_globals
+# @INTERNAL
+# @DESCRIPTION:
+# Set ebuild metadata variables.
+_app-alternatives_set_globals() {
+	debug-print-function ${FUNCNAME} "${@}"
+
+	if [[ ${ALTERNATIVES@a} != *a* ]]; then
+		die 'ALTERNATIVES must be an array.'
+	elif [[ ${#ALTERNATIVES[@]} -eq 0 ]]; then
+		die 'ALTERNATIVES must not be empty.'
+	fi
+
+	HOMEPAGE="https://wiki.gentoo.org/wiki/Project:Base/Alternatives"
+	S=${WORKDIR}
+
+	LICENSE="CC0-1.0"
+	SLOT="0"
+
+	# yep, that's a cheap hack adding '+' to the first flag
+	IUSE="+${ALTERNATIVES[*]%%:*}"
+	REQUIRED_USE="^^ ( ${ALTERNATIVES[*]%%:*} )"
+	RDEPEND=""
+
+	local flag dep
+	for flag in "${ALTERNATIVES[@]}"; do
+		[[ ${flag} != *:* ]] && die "Invalid ALTERNATIVES item: ${flag}"
+		dep=${flag#*:}
+		flag=${flag%%:*}
+		RDEPEND+="
+			${flag}? ( ${dep} )
+		"
+	done
+}
+_app-alternatives_set_globals
+
+# @FUNCTION: get_alternative
+# @DESCRIPTION:
+# Get the flag name for the selected alternative (i.e. the USE flag set).
+get_alternative() {
+	debug-print-function ${FUNCNAME} "${@}"
+
+	local flag
+	for flag in "${ALTERNATIVES[@]%%:*}"; do
+		usev "${flag}" && return
+	done
+
+	die "No selected alternative found (REQUIRED_USE ignored?!)"
+}
+
+fi