Modern Haskell → C translation approaches?

GHC’s “unregisterised” C backend should still work. However, you don’t pass -fvia-C option to GHC – GHC itself needs to be built for the C backend. This means, you need to build GHC yourself.

The -fvia-C option was used for the “registerised” C backend that existed on GHC <= 7.0.

If the compatibility with GHC is not important, you might want to try MicroHs, which bootstraps with a C compiler according to its document.