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

[Ross Paterson]

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)