emscripten: generate libembind*.a

      # wasm2c doesn't work with PIC

      $out/bin/emcc -s WASM2C -s STANDALONE_WASM $LTO test.c

      for BIND in "" "--bind"; do

        for MT in "" "-s USE_PTHREADS"; do

          for RELOCATABLE in "" "-s RELOCATABLE"; do

          $out/bin/emcc $RELOCATABLE $MT $LTO test.c

            $out/bin/emcc $RELOCATABLE $BIND $MT $LTO test.c

          done

        done

      done

    done