wasm: force TCL select() notifier (Emscripten has no working epoll)

TCL 9.1's configure defines NOTIFIER_EPOLL whenever <sys/epoll.h> is
present (unix/configure.ac:342).  Emscripten's sysroot ships that header
and the epoll_* symbols, so TCL picked the epoll notifier — but epoll
does not work under WASM at runtime and the event loop hangs.

Hide the header/functions from TCL's configure via autoconf cache vars
(ac_cv_header_sys_epoll_h=no + epoll_create/epoll_create1=no) so the
select()-based notifier is selected instead.

Verified with emsdk 6.0.3: NOTIFIER_EPOLL is absent from DEFS, the
tclEpollNotfy.o/tclKqueueNotfy.o objects are empty stubs (333 B) and
tclSelectNotfy.o is the live notifier (~9.9 KB); the tcl WASM variant
links (magic.js + magic.wasm) via npm/build.sh out-of-source.

Co-Authored-By: Claude Opus 4.8 (1M context) <[email protected]>

toolchains/emscripten/build-tcl-wasm.sh

@@ -107,7 +107,15 @@ cd "$OUT"
 # patching the read-only TCL source tree.
 if [ ! -f Makefile ]; then
   echo "=== emconfigure ==="
+  # Force the select()-based notifier: Emscripten exposes <sys/epoll.h> and the
+  # epoll_* symbols in its sysroot, so TCL's configure would pick the epoll
+  # notifier — but epoll does not work under WASM at runtime and the event loop
+  # hangs.  Hide the header/functions from configure (autoconf cache vars) so
+  # TCL falls back to select(), which Emscripten does support.
   CFLAGS="-O2 -sUSE_ZLIB=1" \
+  ac_cv_header_sys_epoll_h=no \
+  ac_cv_func_epoll_create=no \
+  ac_cv_func_epoll_create1=no \
     emconfigure "$TCL_SRC/unix/configure" \
       --disable-shared \
       --disable-load \