Alec Theriault authored Jan 07, 2019 and Ben Gamari committed Jan 16, 2019

Summary:
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).

Reviewers: angerman, alpmestan, adamse, DavidEichmann, bgamari, Phyx

Subscribers: rwbarton, carter

GHC Trac Issues: #15990

Differential Revision: https://phabricator.haskell.org/D5409