Use build_script step in AppVeyor configuration

Preparations for building with multiple build systems

Author: adrianbroher

appveyor.yml

@@ -9,7 +9,5 @@ install:
 - git clone -q https://github.com/xiph/ogg.git %APPVEYOR_BUILD_FOLDER%\..\libogg
 - msbuild "%APPVEYOR_BUILD_FOLDER%\..\libogg\win32\VS2015\libogg_static.sln" /m /v:minimal /logger:"C:\Program Files\AppVeyor\BuildAgent\Appveyor.MSBuildLogger.dll" /property:Configuration=%CONFIGURATION%;Platform=%PLATFORM%
 
-build:
-  project: win32\VS2010\vorbis_dynamic.sln
-  parallel: true
-  verbosity: minimal
+build_script:
+- msbuild "%APPVEYOR_BUILD_FOLDER%\win32\VS2010\vorbis_dynamic.sln" /m /v:minimal /logger:"C:\Program Files\AppVeyor\BuildAgent\Appveyor.MSBuildLogger.dll" /property:Configuration=%CONFIGURATION%;Platform=%PLATFORM%