This fixes some of the issues that surfaced when trying to build
dynamic GHC on OSX. Unfortunately, due some other libffi
issues, this doesn't completely fix dynamic builds on OSX.
- Use 'extra-libraries' from .cabal files instead of hardcoding which packages need which extra libs. Also add support for 'extra-lib-dirs'.
- Make sure Hadrian looks in the right places to support both plain '<pkg>.buildinfo' and '<pkg>.buildinfo.in' files.
- Make the '-rpath' support more robust across OS's (it previously didn't work on OSX and possibly windows either).