wasm/README.md: add manual config.guess download and autoconf steps

Autoconf distributed with Ubuntu 22.04 is very old and doesn't support
WASI as an OS, so add instructions to download the latest config.guess,
then run `./autogen.sh`.

See also: 2297012efd

wasm/README.md

@@ -23,7 +23,14 @@ $ export WASI_SDK_PATH=/path/to/wasi-sdk-X.Y
 ```console
 $ export PATH=path/to/binaryen:$PATH
 ```
-5. Configure
+5. Download the latest `config.guess` with WASI support, and run `./autogen.sh` to generate configure when you
+   are building from the source checked out from Git repository
+```console
+$ ruby tool/downloader.rb -d tool -e gnu config.guess config.sub
+$ ./autogen.sh
+```
+
+6. Configure
   - You can select which extensions you want to build.
   - If you got `Out of bounds memory access` while running the produced ruby, you may need to increase the maximum size of stack.
 ```console
@@ -34,7 +41,7 @@ $ ./configure LDFLAGS="-Xlinker -zstack-size=16777216" \
   --with-ext=ripper,monitor
 ```
 
-6. Make
+7. Make
 ```console
 $ make install
 ```