Fix AWS SDK build error on some buildbot builders.

Use older version of the SDK generally, because the newer ones break
with older cmake.

On Macs, use newer version, to fix mac specific the build error.

plugin/aws_key_management/CMakeLists.txt

@@ -108,11 +108,19 @@ ELSE()
     SET_TARGET_PROPERTIES(${lib} PROPERTIES IMPORTED_LOCATION ${loc})
   ENDFOREACH()
 
+  # To be compatible with older cmake, we use older version of the SDK
+  # We increase the version for macs however, so the newest mac could built it.
+  IF(APPLE)
+    SET(GIT_TAG "1.0.100")
+  ELSE()
+    SET(GIT_TAG "1.0.8")
+  ENDIF()
+
   SET(AWS_SDK_PATCH_COMMAND )
   ExternalProject_Add(
     aws_sdk_cpp
     GIT_REPOSITORY "https://github.com/awslabs/aws-sdk-cpp.git"
-    GIT_TAG "1.0.100"
+    GIT_TAG ${GIT_TAG}
     UPDATE_COMMAND ""
     SOURCE_DIR "${CMAKE_CURRENT_BINARY_DIR}/aws-sdk-cpp"
     ${byproducts}