This AppVeyor configuration will do the following:

1. Download MinGW 4.8.5, if it doesn't already have it.
2. Build and install the entire project with MinGW.
3. Build and install everything that MSVC can build, using the MinGW external
   capnp tools.
4. Build the contents of c++/samples.
5. Zip up the contents of each install directory, which will be available for
   download as artifacts.

AppVeyor will autoconfigure itself if it sees this file, so you should
just have to:

1. Log into ci.appveyor.com. You can use a GitHub account for this.
2. Select "New Project."
3. Select capnproto from the list of GitHub projects.

Push to any branch, and AppVeyor should automatically build the new
commit. If you want to build a branch that you've already uploaded, you
can change the default branch in 'Settings -> General', then start a
manual build.

I used Dr. MinGW's appveyor.yml as a template for the MinGW download and
extraction code:
https://github.com/jrfonseca/drmingw/blob/2340928f5cbc0ba718e8dd160e027af7722383c8/appveyor.yml