Modernize `process.cabal`

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>
Make compatible with GHC 7.2/7.4/7.6 again

Supporting GHC 7.2 again was just a single `#ifdef`. Otoh, supporting
GHC 7.0 would require reverting part of d10b168a65701.

This also updates the build-dependancy version constraints to reflect
the current reality. The code is `-Wall` clean for GHC 7.4 and later.

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