Add workaround for VS2015 so that appveyor works

[uucidl]

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