Add cmake as optional build system to AppVeyor configuration

Author: adrianbroher

appveyor.yml

@@ -8,10 +8,29 @@ platform:
 environment:
   matrix:
   - BUILD_SYSTEM: MSVC
+  - BUILD_SYSTEM: CMAKE
+
+matrix:
+  allow_failures:
+  - BUILD_SYSTEM: CMAKE
 
 install:
 - git clone -q https://github.com/xiph/ogg.git %APPVEYOR_BUILD_FOLDER%\..\libogg
 - 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%
+- if "%BUILD_SYSTEM%"=="CMAKE" mkdir "%APPVEYOR_BUILD_FOLDER%\..\libogg\build"
+- if "%BUILD_SYSTEM%"=="CMAKE" pushd "%APPVEYOR_BUILD_FOLDER%\..\libogg\build"
+- if "%BUILD_SYSTEM%"=="CMAKE" cmake -A "%PLATFORM%" -G "Visual Studio 14 2015" ..
+- if "%BUILD_SYSTEM%"=="CMAKE" cmake --build . --config "%CONFIGURATION%" -- /logger:"C:\Program Files\AppVeyor\BuildAgent\Appveyor.MSBuildLogger.dll"
+- if "%BUILD_SYSTEM%"=="CMAKE" popd
+
+before_build:
+- if "%BUILD_SYSTEM%" == "CMAKE" mkdir "%APPVEYOR_BUILD_FOLDER%\build"
+- if "%BUILD_SYSTEM%" == "CMAKE" pushd "%APPVEYOR_BUILD_FOLDER%\build"
+- if "%BUILD_SYSTEM%" == "CMAKE" cmake -A "%PLATFORM%" -G "Visual Studio 14 2015" ..
+- if "%BUILD_SYSTEM%" == "CMAKE" popd
 
 build_script:
 - 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%
+- if "%BUILD_SYSTEM%" == "CMAKE" pushd "%APPVEYOR_BUILD_FOLDER%\build"
+- if "%BUILD_SYSTEM%" == "CMAKE" cmake --build . --config "%CONFIGURATION%" -- /logger:"C:\Program Files\AppVeyor\BuildAgent\Appveyor.MSBuildLogger.dll"
+- if "%BUILD_SYSTEM%" == "CMAKE" popd