message: download_manager: add -frtti flag
  The -frtti flag is required in order to be able to use
  std::dynamic_pointer_cast.
diff: @@ -8,7 +8,7 @@ set(CMAKE_CXX_FLAGS_DEBUG "-DDEBUG")
  # We want C++11 and lots of warnings.
  if(NOT MSVC)
  # Clang and GCC
  - add_definitions("-std=c++11 -Wall -Wextra -Werror")
  + add_definitions("-std=c++11 -Wall -Wextra -Werror -frtti")
  set(platform_libs "pthread")
  else()
  # MSBuild
  