message: build_system: add define for Linux as well
  This is needed for some ifdefs in the serial connection.
diff: @@ -38,6 +38,10 @@ if(APPLE)
  add_definitions("-DAPPLE")
  endif()
  +if(LINUX)
  + add_definitions("-DLINUX")
  +endif()
  +
  # Add DEBUG define for Debug target because that is not done automatically.
  set(CMAKE_CXX_FLAGS_DEBUG "${CMAKE_CXX_FLAGS_DEBUG} -DDEBUG")
  