Add cmake as build system to AppVeyor configuration
Add cmake as build system to AppVeyor configuration

* Use build_script step in AppVeyor configuration.  This is a
  preparation for building with multiple build systems.
* Add BUILD_SYSTEM env variable to AppVeyor build matrix.  This
  allows to switch between different build systems when building
  multiple test matrix configurations.

Signed-off-by: Ralph Giles <65585f72bf62a11d2f4105aa30c20fe94efea46f@thaumas.net>

ShiftMediaProject/ogg,xiph/ogg,xiph/ogg,ShiftMediaProject/ogg