Enable new warning for fragile/incorrect CPP #if usage

The C code in the RTS now gets built with `-Wundef` and the Haskell code
(stages 1 and 2 only) with `-Wcpp-undef`. We now get warnings whereever
`#if` is used on undefined identifiers.

Test Plan: Validate on Linux and Windows

Reviewers: austin, angerman, simonmar, bgamari, Phyx

Reviewed By: bgamari

Subscribers: thomie, snowleopard

Differential Revision: https://phabricator.haskell.org/D3278