There's a few ways to do this. Which one is "best" for you depends on how technical you expect your audience to be.
The easiest way for you is probably to just tell your users that you use some or another library and that they need to have it installed.
The easiest way for your users is to copy/paste the parts of the library that you use into your code. The threads library you linked is CC0 so you can do that. This also futureproofs your design against changes that break this library and against this library becoming unavailable. (Normally copy/pasting from libraries is discouraged as this futureproofs designs against improvements in the library as well, but IMHO certain OpenSCAD libraries should be considered an exception to this rule. If a library renders the shapes that you want fast enough to make improvements irrelevant, there's no need to keep the library external.)