This includes declaring all potentially used language pragmas in
`other-extensions` and declare the `CPP` in the `.hs` files instead of
relying on `default-extensions`.

The confusing NHC98/HUGS conditionals should be removed, as the package
most probably won't work w/ those compilers anyway these days.

Signed-off-by: Herbert Valerio Riedel <hvr@gnu.org>