emscripten: clean up `install` method
emscripten: clean up `install` method

1. Use the `install_prefix` kwarg for `std_cmake_args`.
2. Avoid hardcoding the executables we pass to `write_env_script`.
3. Don't pass `CMAKE_LINKER` to the LLVM build.
4. Invoke `make` through `cmake` when building `binaryen`.
5. Don't use `ENV.cxx11`.

chrisfinazzo/homebrew-core,adam-moss/homebrew-core,zyedidia/homebrew-core,sjackman/homebrew-core,zyedidia/homebrew-core,nbari/homebrew-core,cblecker/homebrew-core,lemzwerg/homebrew-core,makigumo/homebrew-core,uyjulian/homebrew-core,nandub/homebrew-core,chrisfinazzo/homebrew-core,nbari/homebrew-core,Homebrew/homebrew-core,Homebrew/homebrew-core,filcab/homebrew-core,filcab/homebrew-core,sjackman/homebrew-core,stgraber/homebrew-core,cblecker/homebrew-core,adam-moss/homebrew-core,nandub/homebrew-core,makigumo/homebrew-core,lemzwerg/homebrew-core,stgraber/homebrew-core,uyjulian/homebrew-core