Appveyor: we need to copy the latest Git commit, not check out the super-project which may be out of date!

appveyor.yml

@@ -28,8 +28,8 @@ install:
   - cd ..
   - git clone -b %APPVEYOR_REPO_BRANCH% --depth 1 https://github.com/boostorg/boost.git boost-root
   - cd boost-root
+  - xcopy /s /e /q %APPVEYOR_BUILD_FOLDER% libs\config
   - git submodule update --init tools/build
-  - git submodule update --init libs/config
   - git submodule update --init tools/boostdep
   - python tools/boostdep/depinst/depinst.py config
   - bootstrap