Suppress missing haddock warning for some packages.

Allow a hardcoded whitelist of packages (currently 'rts') to not have
haddocks. When building several packages together, the accumulation of
these spurious warnings can make it a bit harder to notice legitimate
errors.
Rename a lamda var to avoid shadow warning.
Merge pull request #1231 from kowey/kowey

suppress annoying missing haddock warning for whitelist (rts)
Relax unix dep
Add some support for libraries that make multiple .dll/.so/.a/... files

Cabal doesn't know how to build them, but it does know how to install
and register them. In particular, this means that GHC's build system
can use Cabal to install them.
Comment.