It seems Appveyor's images have been changed. The builds could not
find standard library headers anymore.

It seems to be an interaction between MS' scripts and the Windows
Driver Kit that is installed there.

One way to disable this interaction is to rename the `wdf` folder to
some other name.

See:
- http://stackoverflow.com/questions/31862627/vs2015-cl-cant-find-crt-libs-stido-h-ctype-h-etc-when-building-on-command-l
- http://help.appveyor.com/discussions/problems/3062-problem-with-new-updates-visual-studio-2015-and-cmake
- https://github.com/appveyor/ci/issues/414