* guix/build/haskell-build-system.scm (%standard-phases): move 'haddock phase
before 'install phase.
* guix/build/haskell-build-system.scm (haddock): Simplify it as the 'install
phase takes care of copying files.
* guix/build/haskell-build-system.scm (configure): Add '--libsubdir' flag.
Fix use of '--extra-include-dirs' and '--extra-lib-dirs' flags. Use 'doc',
'bin' and 'lib' outputs if they are defined.
* guix/build/haskell-build-system.scm (make-ghc-package-database, register):
Aligh location of 'package.conf.d' directory with '--libsubdir' flag.