configure.ac: don't use shutdown on emscripten

... to absorb a change on Ubuntu 21.10

configure.ac

@@ -1204,6 +1204,7 @@ main()
 		RUBY_APPEND_OPTIONS(LDFLAGS, "-sALLOW_MEMORY_GROWTH=1")
 		RUBY_APPEND_OPTIONS(LDFLAGS, "-sASYNCIFY")
 		RUBY_APPEND_OPTIONS(LDFLAGS, "-sFORCE_FILESYSTEM=1")
+		ac_cv_func_shutdown=no
 		],
 [	LIBS="-lm $LIBS"])
 : ${ORIG_LIBS=$LIBS}