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/Manifest

@@ -1,3 +1,3 @@
 DIST abseil-cpp-20200225.2.tar.gz 1679650 BLAKE2B bbadb2b86fd390f51322e687526c72a3b13caff6391974462a1146cc563616ce479330336a489a71e27b143e7cbea6695fc12c91fb721185a5116b3a6c5506b1 SHA512 75a607dee825e83c10dcd5e509515461f1b12c4aca861e4739ac4d41357b8e893dbfbe33873aa5c05463dde0891dedd7535af2ec59f173de29488e1b1321b335
-DIST abseil-cpp-20200923.1.tar.gz 1783899 BLAKE2B 2f9f07bdfd5fe5b70bd470446cf00a3f6ce7790a60bf5063120fb00e78c73f19ddb7843bb57079c64a9542953b121dbac6cf0cef88617effc6e74344711142bd SHA512 1fb484f66218a2296e9f648ed98a5d09e591eb680ea1912cce44be22ea8051a0d0e27ab30f5df1ec22311c11768d4d998bbdfc62a1ef6c0088e254dd19c71f7b
+DIST abseil-cpp-20200923.2.tar.gz 1783972 BLAKE2B 7ba01cab353de9bf34230e0efa567b1dc16680144579841a0fd127b95fb36cc8b63fc3c7da6a0a7c350c32b91955c6b6bb23a3ca52e50174a4dce5d3c1b9e936 SHA512 fdbb0f84265bc1d4f24aac0af6d0eb8cb603f801fc14d63ac1e9d3bab47703a8a2ff5f44a0a4d325819dbea50fc533d967e1bfd65004ec11ef35fadc44a2b855
 DIST gtest-1.10.0_p20200702.tar.gz 866900 BLAKE2B c162d47868583ba270675abe5df935b8f4b9a9c00dbdc80dd88afa7f2b98266640b32b2ea5e8f9f6d6227196b5d2f79dbfda4e9585106224e814cc1503cf777d SHA512 715d887b59b47d4691c7c90ef0cf0ffc3d1e758e500263c76b50fd506e90a9d1c390af745933cfe3f55e1edac5d72dccedef3cb9a50b71a5b796424471a3017b

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

@@ -22,7 +22,7 @@ LICENSE="
 "
 SLOT="0"
 KEYWORDS="~amd64 ~ppc64 ~x86"
-IUSE="test"
+IUSE="cxx17 test"
 
 DEPEND=""
 RDEPEND="${DEPEND}"
@@ -57,6 +57,7 @@ src_configure() {
 		-DABSL_ENABLE_INSTALL=TRUE
 		-DABSL_LOCAL_GOOGLETEST_DIR="${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

dev-cpp/abseil-cpp/metadata.xml

@@ -5,4 +5,7 @@
 		<email>gyakovlev@gentoo.org</email>
 		<name>Georgy Yakovlev</name>
 	</maintainer>
+	<use>
+		<flag name="cxx17">Build with -DCMAKE_CXX_STANDARD=17 (required by some revdeps)</flag>
+	</use>
 </pkgmetadata>