zxing-cpp: fix darwin build

pkgs/development/libraries/zxing-cpp/default.nix

@@ -21,6 +21,9 @@ stdenv.mkDerivation rec {
     pkg-config
   ];
 
+  # error: 'path' is unavailable: introduced in macOS 10.15
+  CXXFLAGS = lib.optional (stdenv.hostPlatform.system == "x86_64-darwin") "-D_LIBCPP_DISABLE_AVAILABILITY";
+
   cmakeFlags = [
     "-DBUILD_EXAMPLES=OFF"
   ];