Back-port Haskell-related improvements from stdenv-updates.

 * There now is full support for building Haskell packages as shared libraries
   for GHC versions 7.4.2 or later. The Cabal builder recognizes the following
   attributes:

    - enableSharedLibraries configures Cabal to build of shared libraries in
      addition to static ones. This option requires that all dependencies of
      the package have been compiled for use in shared libraries, too.

    - enableSharedExecutables configures Cabal to prefer shared libraries when
      linking executables.

   The default values for these attributes are arguments to the haskellPackages
   expression.

 * Haskell builds now run in a LANG="en_US.UTF-8" environment to avoid plenty
   of build and test suite errors. Without this setting, GHC seems unable to
   deal with the UTF-8 character encoding that's generally considered standard
   in the Haskell world.

 * The Cabal builder supports a new attribute 'testTarget' to specify the exact
   set of tests to be run during the check phase.

 * The ghc-wrapper attribute ghcVersion has been removed. Instead, we use the
   ghc.version attribute, which exists in unwrapped GHC derivations, too.