message: Improve protobuf CMakeLists now that we don't support Ubuntu 16 anymore
diff: @@ -33,18 +33,10 @@ foreach(CMAKE_ARG ${CMAKE_ARGS})
  message(STATUS "-- ${CMAKE_ARG}")
  endforeach()
  -# This wouldn't be necessary with CMAKE_ARGS + SOURCE_SUBDIR (see below)
  -if(DEFINED CMAKE_GENERATOR)
  - set(CMAKE_GENERATOR_PARAM "-G${CMAKE_GENERATOR}")
  -endif()
  -
  ExternalProject_add(
  protobuf
  URL https://github.com/protocolbuffers/protobuf/releases/download/v3.13.0/protobuf-cpp-3.13.0.tar.gz
  PREFIX protobuf
  - CONFIGURE_COMMAND ${CMAKE_COMMAND} "${CMAKE_GENERATOR_PARAM}" "${CMAKE_ARGS}" <SOURCE_DIR>/cmake
  - # CMAKE_ARGS + SOURCE_SUBDIR are more elegant than CONFIGURE_COMMAND,
  - # but not supported by CMake 3.5.1 (Ubuntu 16.04)
  - #CMAKE_ARGS "${CMAKE_ARGS}"
  - #SOURCE_SUBDIR cmake
  + CMAKE_ARGS "${CMAKE_ARGS}"
  + SOURCE_SUBDIR cmake
  )
  