The warning about rtsopts having no consequences is expected.
So accept new stderr.

We used to filter rtsopts, i opted to instead just accept the warning of it having no effect.
This works both for -rtsopts, as well as -with-rtsopts which winio adds.

Demand cabal 2.0 syntax instead of >= 1.20 as required by newer cabal versions.

GHC now reports String instead of [Char].

It now explains users may not use these functions when
using the old IO manager.