Add appveyor.yml

This file defines the AppVeyor CI (appveyor.com) settings.

It builds using make+bash under MSYS2 so that the current build system
can be used with minimal changes.  It currently only builds configurator
and generates config.h.

The build and test commands for more thorough testing are left as
comments in appveyor.yml so interested parties can use them as a
starting point for future work.

Note that several compiler errors not related to configurator are
printed due to make attempting to generate and include test-depends.
Although Windows-specific code could be added to Makefile to avoid
these, it seemed unwarranted if the compile errors may be fixed soon.

Changes since v2:
- Add reference to AppVeyor results for canonical repo and basic
  instructions to setup AppVeyor for forks.

Signed-off-by: Kevin Locke <kevin@kevinlocke.name>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>