libwebsockets

appveyor: stop default vc build

Suddenly appveyor just says this

Build started
git clone -q --branch=master https://github.com/warmcat/libwebsockets.git C:\projects\libwebsockets
git checkout -qf e035ceb36e0b79e85bc18b313e8f46eeadd360b6
The build phase is set to "MSBuild" mode (default), but no Visual Studio project or solution files were found in the root directory. If you are not building Visual Studio project switch build mode to "Script" and provide your custom build command.