dev-libs/mpdecimal: Add from Gentoo

It's from Gentoo commit 08a99d6d23ddb4b387293e6501303f26b1484189.

sdk_container/src/third_party/portage-stable/dev-libs/mpdecimal/Manifest

@@ -0,0 +1,2 @@
+DIST dectest.zip 791733 BLAKE2B ba74ee96f6687ce34cac8406fd7fd1f743d72647a5f883f1054829d992d4dc0f61c9e3b88e2529f425cc2fbda14dfc372b3fbfafe6553baabb3244172fed712f SHA512 05f9d12aec1ebfc9ca211b01705b41c5cd1bbc54d9aa6c4799b40fb3ab1835433fd1a277735d26d2530c3793c58b107b3abb1c31f64340e5e9d7856d92c821cf
+DIST mpdecimal-4.0.0.tar.gz 315325 BLAKE2B aed5b6d5c03b12017989bbac9a166366e800ce5ee8d2fad5a1b81def48877126233998df306968902c728776616b1c06ce0ef05ea066c07daf841a53541a09f9 SHA512 7610ac53ac79f7a8a33fa7a3e61515810444ec73ebca859df7a9ddc18e96b990c99323172810c9cc7f6d6e1502c0be308cd443d6c2d5d0c871648e4842e05d59

sdk_container/src/third_party/portage-stable/dev-libs/mpdecimal/metadata.xml

@@ -0,0 +1,8 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<!DOCTYPE pkgmetadata SYSTEM "https://www.gentoo.org/dtd/metadata.dtd">
+<pkgmetadata>
+	<maintainer type="person">
+		<email>mgorny@gentoo.org</email>
+		<name>Michał Górny</name>
+	</maintainer>
+</pkgmetadata>

sdk_container/src/third_party/portage-stable/dev-libs/mpdecimal/mpdecimal-4.0.0.ebuild

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+# Copyright 2024 Gentoo Authors
+# Distributed under the terms of the GNU General Public License v2
+
+EAPI=8
+
+DESCRIPTION="Correctly-rounded arbitrary precision decimal floating point arithmetic"
+HOMEPAGE="https://www.bytereef.org/mpdecimal/"
+SRC_URI="
+	https://www.bytereef.org/software/mpdecimal/releases/${P}.tar.gz
+	test? (
+		https://speleotrove.com/decimal/dectest.zip
+	)
+"
+
+LICENSE="BSD-2"
+SLOT="$(ver_cut 1)"
+KEYWORDS="~alpha amd64 arm arm64 hppa ~ia64 ~loong ~m68k ~mips ppc ppc64 ~riscv ~s390 sparc x86"
+IUSE="cxx test"
+RESTRICT="!test? ( test )"
+
+BDEPEND="
+	test? (
+		app-arch/unzip
+	)
+"
+
+src_unpack() {
+	unpack "${P}.tar.gz"
+	if use test; then
+		mkdir "${P}/tests/testdata" || die
+		cd "${P}/tests/testdata" || die
+		unpack dectest.zip
+	fi
+}
+
+src_prepare() {
+	default
+
+	# sigh
+	sed -i -e "s:/lib:/$(get_libdir):" lib*/.pc/*.pc.in || die
+}
+
+src_configure() {
+	local myconf=(
+		# just COPYRIGHT.txt
+		--docdir=/removeme
+		$(use_enable cxx)
+	)
+
+	# more sigh
+	# https://bugs.gentoo.org/931599
+	local -x LDXXFLAGS="${LDFLAGS}" LD="${CC}" LDXX="${CXX}"
+	econf "${myconf[@]}"
+}
+
+src_test() {
+	emake check
+}
+
+src_install() {
+	default
+	rm -r "${D}/removeme" || die
+}