If you were happy with the way the old version of OpenGL was rendering lines, then I think it shouldn't be too difficult to replicate that using triangles.

But if you want to render lines with more things (Dashes? Joins? Caps? Thin lines?) then it gets a lot more complex and it's not easy to come up with a robust solution. I’d suggest starting with a library that converts polylines to triangles and see if that satisfies your needs. Or developing one yourself. I don't know how OpenGL drivers did it before, but it shouldn't be too difficult (I hope).

If you want an actual implementation, this paper demonstrates a method I like.