hadrian: Disable -Wno-undef in files which include ffi.h (#459)
hadrian: Disable -Wno-undef in files which include ffi.h (#459)

As pointed out in D4181, these headers refer to undefined macros.
snowleopard/hadrian,sdiehl/ghc,bgamari/shaking-up-ghc,sdiehl/ghc,snowleopard/shaking-up-ghc,sdiehl/ghc,sdiehl/ghc,sdiehl/ghc,sdiehl/ghc,sdiehl/ghc