#140 (Cabal doesn't allow doc/html/<package name> to be overridden)

igloo writes:

Distribution.Simple.LocalBuildInfo uses

haddockPref pkg_descr = foldl1 () [distPref, "doc", "html", pkgName (package pkg_descr)]

and provides no way to override it. This should be a new field docdir (or similar), and be overridable with a configure flag.

haddockPref determines where the documentation is built; mkHaddockDir says where it is installed, currently $datadir/$datasubdir/doc/html/$pkg, where $datasubdir defaults to $pkgid. That default doesn't seem right in any case. Perhaps we want --docdir (defaulting to $prefix/share/doc on Unix and something else on Windows) --htmlsubdir (defaulting to $pkgid/html)