message: CMake: fix Emscripten build.
  Instead of setting things in CMake itself, the toolchain file should be provided via `-DCMAKE_TOOLCHAIN_FILE`.
diff: @@ -5,9 +5,12 @@ if (SPIRIT_BUILD_FOR_JS)
  ###
  MESSAGE( STATUS ">> Choosing compiler: emcc" )
  ### Set the path to emscripten
  - SET(EMSCRIPTEN_ROOT_PATH "/usr/local/emsdk/emscripten/1.38.29/")
  + # SET(EMSCRIPTEN_ROOT_PATH "/usr/local/emsdk/emscripten/1.38.29/")
  ### Use the Emscripten toolchain file
  - SET(CMAKE_TOOLCHAIN_FILE Emscripten)
  + # SET(CMAKE_TOOLCHAIN_FILE /usr/local/emsdk/emscripten/1.38.13/cmake/Modules/Platform/Emscripten)
  + ###
  + MESSAGE( STATUS ">> EMSCRIPTEN_ROOT_PATH: \"${EMSCRIPTEN_ROOT_PATH}\"")
  + MESSAGE( STATUS ">> CMAKE_TOOLCHAIN_FILE: \"${CMAKE_TOOLCHAIN_FILE}\"")
  ######################################################################
  