Libc implementations have a very clear scope, clear enough that you can point to the specification. That is their 'one thing' do what that spec says.
Eclipse however, doesn't have that singular goal. You would be hard pressed to say how many of Eclipse's tentacles is a clear push towards being an ide. What should a completely finished version of Eclipse that met all it's goals look like?
Similarly the one thing could be "be a c preprocessor" or be a full "c compiler" these are both "one thing"s even while one is a subset of the other.
The intention of "do one thing, and do it well" is not to limit the scope but to show the boundary of the scope and to commit to doing everything within that boundary
By making your one thing "a full c compiler" you should be committing to doing everything that someone making a c preprocessor is doing, and to the same standard. The Unix philosophy should be considered a warning not to neglect components because you are working on a larger system.
You can't do everything, but you don't have to. If others are following the same principles then many of the parts of what you need will be done to a high standard by others.
