$OpenBSD$

It's gtimeout, from GNU coreutils.

Index: src/build.lua
--- src/build.lua.orig
+++ src/build.lua
@@ -163,7 +163,7 @@ function simpletest(interpreter, e)
 	rule {
 		ins = e.ins,
 		outs = { badfile },
-		cmd = "timeout 5s "..interpreter.." @1 > &1 && diff -u -w &1 "..e.goodfile,
+		cmd = "gtimeout 5s "..interpreter.." @1 > &1 && diff -u -w &1 "..e.goodfile,
 	}
 end
 
