Control CPP through settings file (#8683)

Summary:
Allow the CPP program and flag choices for GHC
be configured via the the ghc settings file

Test Plan: ran validate yesterday

Reviewers: hvr, austin, mzero, simonmar

Reviewed By: austin, mzero, simonmar

Subscribers: mzero, simonmar, relrod, carter

Differential Revision: https://phabricator.haskell.org/D26

(cherry picked from commit 34f7e9a3)