down grade protobuf to 3.1

cmake/external/protobuf.cmake

@@ -199,7 +199,7 @@ FUNCTION(build_protobuf TARGET_NAME BUILD_FOR_HOST)
     ENDIF()
 
     SET(PROTOBUF_REPOSITORY  https://github.com/protocolbuffers/protobuf.git)
-    SET(PROTOBUF_TAG         2761122b810fe8861004ae785cc3ab39f384d342)
+    SET(PROTOBUF_TAG         9f75c5aa851cd877fb0d93ccc31b8567a6706546)
 
     cache_third_party(${TARGET_NAME}
         REPOSITORY    ${PROTOBUF_REPOSITORY}
@@ -234,7 +234,7 @@ FUNCTION(build_protobuf TARGET_NAME BUILD_FOR_HOST)
     )
 ENDFUNCTION()
 
-SET(PROTOBUF_VERSION 3.5.0)
+SET(PROTOBUF_VERSION 3.1.0)
 
 IF(NOT PROTOBUF_FOUND)
     build_protobuf(extern_protobuf FALSE)