appveyor: use opus artifacts instead of compiling it

.appveyor.yml

@@ -8,11 +8,15 @@ platform:
 - Win32
 - x64
 
+environment:
+  opus_url: https://ci.appveyor.com/api/projects/$(APPVEYOR_ACCOUNT_NAME)/opus/artifacts/opus.zip
+
 install:
 - cd %APPVEYOR_BUILD_FOLDER%\..
-- git clone -q https://github.com/xiph/opus.git opus
+- 'curl -LOG --data-urlencode "job=Configuration: %CONFIGURATION:-NoHTTP=%; Platform: %PLATFORM%" %OPUS_URL%'
+- 7z x opus.zip -oopus-artifacts
+- move /Y opus-artifacts opus
 - git clone -q https://github.com/xiph/ogg.git ogg
-- msbuild opus\win32\VS2015\opus.vcxproj /p:Configuration=%CONFIGURATION:-NoHTTP=%;Platform=%PLATFORM% /m /v:minimal /logger:"C:\Program Files\AppVeyor\BuildAgent\Appveyor.MSBuildLogger.dll"
 - msbuild ogg\win32\VS2015\libogg_static.sln /p:Configuration=%CONFIGURATION:-NoHTTP=%;Platform=%PLATFORM% /m /v:minimal /logger:"C:\Program Files\AppVeyor\BuildAgent\Appveyor.MSBuildLogger.dll"
 - if %CONFIGURATION:-NoHTTP=%==%CONFIGURATION% git clone -q --branch=OpenSSL_1_0_2-stable https://github.com/openssl/openssl.git openssl
 - ps: >-