Repair appveyor... Horrifyingly
Repair appveyor... Horrifyingly

It's sort of gnarly that it's still doing in-tree builds, but I
really, _really_ don't want to get any more friendly with CMake's
Visual Studio generator to work out how to make it stop doing it.

In-tree builds still work, after all, and the goal of this work is
to make out-of-tree builds work as well. Notional horrors like
this will have to wait ;)

nlohmann/json,hunter-packages/json,theodelrieu/json,kaidokert/json,stanmihai4/json,whackashoe/json,nlohmann/json,Newbrick/json,kaidokert/json,hunter-packages/json,nlohmann/json,whackashoe/json,stanmihai4/json,robertmrk/json,theodelrieu/json,hunter-packages/json,Newbrick/json,Newbrick/json,kaidokert/json,stanmihai4/json