Add BUILD_SYSTEM env variable to AppVeyor build matrix

This allows to switch between different build systems when building
multiple test matrix configurations.

appveyor.yml

@@ -5,9 +5,13 @@ configuration:
 platform:
 - Win32
 
+environment:
+  matrix:
+  - BUILD_SYSTEM: MSVC
+
 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%
+- if "%BUILD_SYSTEM%"=="MSVC" 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_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%
+- if "%BUILD_SYSTEM%"=="MSVC" 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%