Add ability to force -dynamic flag on custom builds.

**Describe the feature request**
I need to be able to force cabal to use the -dynamic flag on ghc when compiling Setup.hs on custom builds on Arch Linux.

**Additional context**
According to [this](https://stackoverflow.com/questions/58559840/cabal-stack-ignores-ghc-options-for-custom-setup-scripts) it's not possible to do that. Arch compiles Haskell differently and ghc always needs to be called up with the -dynamic flag for it to work.