dev-cpp/abseil-cpp: bump to 20200923.2, add cxx17 useflag

Some revdeps may require absl built with -DCMAKE_CXX_STANDARD=17
Normally that should not be a useflag, but we need to expose it to
package-manager so other packages can depend on it.
Upstream insist on using cmake for that and not proper headers...

Package-Manager: Portage-3.0.8, Repoman-3.0.2
Signed-off-by: Georgy Yakovlev <gyakovlev@gentoo.org>

dev-cpp/abseil-cpp/abseil-cpp-20200923.2.ebuild

@@ -0,0 +1,64 @@
+# Copyright 2020 Gentoo Authors
+# Distributed under the terms of the GNU General Public License v2
+
+EAPI=7
+
+PYTHON_COMPAT=( python3_{6,7,8,9} )
+
+inherit cmake python-any-r1
+
+# yes, it needs SOURCE, not just installed one
+GTEST_COMMIT="aee0f9d9b5b87796ee8a0ab26b7587ec30e8858e"
+GTEST_FILE="gtest-1.10.0_p20200702.tar.gz"
+
+DESCRIPTION="Abseil Common Libraries (C++), LTS Branch"
+HOMEPAGE="https://abseil.io"
+SRC_URI="https://github.com/abseil/abseil-cpp/archive/${PV}.tar.gz -> ${P}.tar.gz
+	https://github.com/google/googletest/archive/${GTEST_COMMIT}.tar.gz -> ${GTEST_FILE}"
+
+LICENSE="
+	Apache-2.0
+	test? ( BSD )
+"
+SLOT="0"
+KEYWORDS="~amd64 ~ppc64 ~x86"
+IUSE="cxx17 test"
+
+DEPEND=""
+RDEPEND="${DEPEND}"
+
+BDEPEND="
+	${PYTHON_DEPS}
+	test? ( sys-libs/timezone-data )
+"
+
+RESTRICT="!test? ( test )"
+
+src_prepare() {
+	cmake_src_prepare
+
+	# un-hardcode abseil compiler flags
+	sed -i \
+		-e '/"-maes",/d' \
+		-e '/"-msse4.1",/d' \
+		-e '/"-mfpu=neon"/d' \
+		-e '/"-march=armv8-a+crypto"/d' \
+		absl/copts/copts.py || die
+
+	# now generate cmake files
+	absl/copts/generate_copts.py || die
+
+	sed -i 's/-Werror//g' \
+		"${WORKDIR}/googletest-${GTEST_COMMIT}"/googletest/cmake/internal_utils.cmake || die
+}
+
+src_configure() {
+	local mycmakeargs=(
+		-DABSL_ENABLE_INSTALL=TRUE
+		-DABSL_LOCAL_GOOGLETEST_DIR="${WORKDIR}/googletest-${GTEST_COMMIT}"
+		-DABSL_RUN_TESTS=$(usex test)
+		$(usex cxx17 -DCMAKE_CXX_STANDARD=17 '') # it has to be a useflag for some consumers
+		$(usex test -DBUILD_TESTING=ON '') #intentional usex
+	)
+	cmake_src_configure
+}