message: CMake of Ubuntu 16.04 does not support SOURCE_SUBDIR
diff: @@ -34,5 +34,6 @@ ExternalProject_add(
  URL https://github.com/protocolbuffers/protobuf/releases/download/v3.7.1/protobuf-cpp-3.7.1.tar.gz
  PREFIX protobuf
  CMAKE_ARGS "${CMAKE_ARGS}"
  - SOURCE_SUBDIR cmake
  + CONFIGURE_COMMAND ${CMAKE_COMMAND} "-G${CMAKE_GENERATOR}" "${CMAKE_ARGS}" <SOURCE_DIR>/cmake
  + #SOURCE_SUBDIR cmake # Not supported by CMake 3.5.1 (Ubuntu 16.04)
  )
  