constraints for dynamic / static executables

Currently the choice for building statically or dynamically linked executables is made by the users. Unfortunately not in all configurations static and dynamic linking are interchangeable: When GHC is built with dynamic libraries, a program using the GHC API to interpret or typecheck Haskell needs to be linked dynamically. This affects at least:
- `GHCJS`
- `ghc-mod`
- snap and yesod

So would be nice to be able to specify constraints on how an executable is linked, and to figure whether GHC uses dynamic libs with a flag.

Start by tracing how Cabal currently selects static versus dynamic executable linking and how it detects whether GHC uses dynamic libraries. Review the GHC API consumers named in the issue—GHCJS, ghc-mod, snap, and yesod—to understand the required constraints. Done means executable-linking constraints can be specified and incompatible configurations are identifiable.

Written by the indexing model from the issue text.