I am not sure if it should go to master or to 1.13.1

I merged to master since I don't feel we should do a patch release for this because:

• the problem is not in this repo but in an upstream break;
• our usage of cxxopts is consistent with bundled version at all times so everything works out of the box for any user following our instructions with submodules.

We're trying to keep things up to date and simple, but if users want to use custom versions of the dependencies, they have to somehow be prepared for custom adjustments.