GH-95952: Require setting HOSTRUNNER when cross compiling for WASI (#138203)

macOS by default doesn't have the requisite CLI tools to make a default value easy. Plus Tools/wasm/wasi takes care of setting HOSTRUNNER.

Author: brettcannon

Misc/NEWS.d/next/Build/2025-08-27-11-32-02.gh-issue-95952.KSymc7.rst

@@ -0,0 +1,5 @@
+When cross-compiling for WASI, require that the HOSTRUNNER environment
+variable be explicitly set.
+
+This was needed as macOS lacks the appropriate CLI tools to set a reasonable
+default.

configure

@@ -1636,10 +1636,9 @@ then
       HOSTRUNNER="$NODE"
       AS_VAR_IF([host_cpu], [wasm64], [AS_VAR_APPEND([HOSTRUNNER], [" --experimental-wasm-memory64"])])
     ],
-    dnl TODO: support other WASI runtimes
-    dnl wasmtime starts the process with "/" as CWD. For OOT builds add the
-    dnl directory containing _sysconfigdata to PYTHONPATH.
-    [WASI], [HOSTRUNNER='wasmtime run --wasm max-wasm-stack=16777216 --wasi preview2=n --env PYTHONPATH=/$(shell realpath --relative-to $(abs_srcdir) $(abs_builddir))/$(shell cat pybuilddir.txt):/Lib --dir $(srcdir)::/'],
+    [WASI], [
+      AC_MSG_ERROR([HOSTRUNNER must be set when cross-compiling to WASI])
+    ],
     [HOSTRUNNER='']
   )
 fi