zbackup: Fix build error

Upstream bug report: https://github.com/zbackup/zbackup/issues/160

pkgs/tools/backup/zbackup/default.nix

···
     ./protobuf-api-change.patch
   ];
 
+  # zbackup uses dynamic exception specifications which are not
+  # allowed in C++17
+  NIX_CFLAGS_COMPILE = [ "--std=c++14" ];
+
   buildInputs = [ zlib openssl protobuf lzo libunwind ];
   nativeBuildInputs = [ cmake protobufc ];