Enable -Wcpp-undef for GHC and runtime system

Authored by bgamari on Aug 18 2017, 7:37 AM.

Description

Enable -Wcpp-undef for GHC and runtime system

This gets us much of the benefit of enabling it globally, which avoiding
(at least for now) the pain of making the core libraries build as well.
See Trac #13636.

Test Plan: Validate

Reviewers: erikd, austin

Subscribers: rwbarton, thomie

GHC Trac Issues: Trac #13636

Differential Revision: https://phabricator.haskell.org/D3517