message: Fix RPi build by updating protobuf
diff: @@ -47,7 +47,9 @@ endif()
  ExternalProject_add(
  protobuf
  - URL https://github.com/protocolbuffers/protobuf/releases/download/v3.7.1/protobuf-cpp-3.7.1.tar.gz
  + GIT_REPOSITORY https://github.com/protocolbuffers/protobuf
  + GIT_TAG ee4f2492ea4e7ff120f68a792af870ee30435aa5
  + #URL https://github.com/protocolbuffers/protobuf/releases/download/v3.7.1/protobuf-cpp-3.7.1.tar.gz
  PREFIX protobuf
  CONFIGURE_COMMAND ${CMAKE_COMMAND} "${CMAKE_GENERATOR_PARAM}" "${CMAKE_ARGS}" <SOURCE_DIR>/cmake
  # CMAKE_ARGS + SOURCE_SUBDIR are more elegant than CONFIGURE_COMMAND,
  