Currently, some thorns within the ET only work on some machines (for good reasons), and are thus disabled by default in the thornlist. This has several problems (they are not checked out by default, nor then synced to machines where they are supported and might be needed). We need to improve this.
We could find a way to tag these as "optional" and then have a corresponding tag per machine that can enable them, if they are present. This way (at least most of them) could always be checked out, but Cactus would skip their compilation if they are not present, or not supported on that machine (indicated by something "missing" in the option list probably).