For Cabal packages with "build-type: Configure", a configure shell
script is run to set up build parameters. These scripts need the
CONFIG_SHELL environment variable to be set to function properly.
* guix/build/haskell-build-system.scm (configure): Set CONFIG_SHELL if
necessary.
A Cabal package is allowed to declare an "empty" library, in an
otherwise executable-only package, for the purpose of allowing Cabal
to use it as a dependency for other packages. See e.g. hspec-discover.
* guix/build/haskell-build-system.scm (register): Unconditionally call
setup script with "register", and install any config file generated.
* 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.
