Invalid configuration `wasm32-unknown-emscripten': system `emscripten' not recognized

I'm building Python 3.8.10 using emscripten. I used the same command line like https://rdb.name/panda3d-webgl.md. But these error accured.
`READELF=true BASECFLAGS="-s FILESYSTEM=0 -s DISABLE_EXCEPTION_THROWING=1 -s DISABLE_EXCEPTION_CATCHING=1" LDFLAGS="-s USE_ZLIB=1 -s DISABLE_EXCEPTION_THROWING=1 -s DISABLE_EXCEPTION_CATCHING=1" emconfigure ./configure --host=wasm32-unknown-emscripten --without-pymalloc --disable-shared --enable-ipv6 --prefix= --build=x86_64-pc-linux-gnu --enable-optimizations configure: ./configure --host=wasm32-unknown-emscripten --without-pymalloc --disable-shared --enable-ipv6 --prefix= --build=x86_64-pc-linux-gnu --enable-optimizations`

```
checking build system type... x86_64-pc-linux-gnu
checking host system type... Invalid configuration `wasm32-unknown-emscripten': system `emscripten' not recognized
configure: error: /bin/bash ./config.sub wasm32-unknown-emscripten failed
emconfigure: error: './configure --host=wasm32-unknown-emscripten --without-pymalloc --disable-shared --enable-ipv6 --prefix= --build=x86_64-pc-linux-gnu --enable-optimizations' failed (returned 1)
```
Does anyone help? Thanks.