Apply fix for appveyor to pass

[jvlmdr]

That was more painful than I expected haha. In the end it seemed like the problem was that std::abs<int>() compiled without a problem but then always returned zero at run-time (with the visual studio compiler). I don't have direct access to a visual studio compiler so it's not easy to check.

This PR is ready to go now. (I propose squashing the commits into one.)

[cheind]

Thanks, Jack. Squash seems a good idea.

[jvlmdr]

Thanks!