Easier debugging when using cmake commands generated by mrdocs

[anarthal]

I've had a hard time figuring out what was going wrong in #761, in part because CMake generated files seem to be deleted as soon as mrdocs return. When things go wrong, it's very difficult to find out what happened.

In particular, I had to invent an ugly workaround to copy compile_commands.json to a location where mrdocs wouldn't delete it.

I think mrdocs shouldn't be trying to hide CMake generated files. They are a valuable tool when things go wrong (e.g. imagine a non-Boost user submitting a bug report).