zxing-cpp: c++20 for char8_t

build fails on clang-19 due to using c++ 17 and having char8_t typedefed
to an uint8_t, which is not supported by std::char_traits. Using c++20
fixes this as the code will use a char8_t.

Used a substituteInPlace so the override can be removed once c++20 is
the default in the cmake file.

pkgs/by-name/zx/zxing-cpp/package.nix

@@ -18,6 +18,12 @@ stdenv.mkDerivation (finalAttrs: {
     hash = "sha256-teFspdATn9M7Z1vSr/7PdJx/xAv+TVai8rIekxqpBZk=";
   };
 
+  # c++ 20 needed for char8_t or clang-19 build fails
+  postPatch = ''
+    substituteInPlace CMakeLists.txt \
+      --replace-fail "CMAKE_CXX_STANDARD 17" "CMAKE_CXX_STANDARD 20"
+  '';
+
   nativeBuildInputs = [
     cmake
     pkg-config