Lem can't find its standard library on Windows

[Timmmm]

See rems-project/linksem#33

Tbh I can't think of a good way to fix this, but just so you know. If you're ever changing how the standard library is found, do it in a way that doesn't rely on embedded absolute paths (which is also bad for things like Nix)!

[Alasdair]

I got pretty far dune-ifying the lem build, but this standard library issue hit me as well. The way it's set up now is a bit odd I think.