message: Added support to choose build mode in command line
  It somehow didn't work with the command `cmake -DCMAKE_BUILD_TYPE=Release`.
diff: @@ -17,7 +17,9 @@ set(CMAKE_CXX_STANDARD 14)
  set(CMAKE_EXE_LINKER_FLAGS "-static-libstdc++")
  # Possible values are Debug, Release, RelWithDebInfo, MinSizeRel
  -set(CMAKE_BUILD_TYPE "Debug")
  +if(NOT CMAKE_BUILD_TYPE)
  + set(CMAKE_BUILD_TYPE "Debug" FORCE)
  +endif(NOT CMAKE_BUILD_TYPE)
  set(CMAKE_ARCHIVE_OUTPUT_DIRECTORY "_build")
  set(CMAKE_LIBRARY_OUTPUT_DIRECTORY "_build")
  