Validate with --hadrian fails to build ghc on windows for GHC-HEAD

Summary

Write a brief description of the issue.

Steps to reproduce

THREADS=4 ./validate --hadrian

eventually errors with

Exit code: 1
Stderr:
compiler\main\DynFlags.hs:323:0: error:
     error: "GHC_STAGE" is not defined, evaluates to 0 [-Werror=undef]
      323 | #if GHC_STAGE >= 2
          |
    |
323 | #if GHC_STAGE >= 2
    | ^

compiler\main\DynFlags.hs:5747:0: error:
     error: "GHC_STAGE" is not defined, evaluates to 0 [-Werror=undef]
     5747 | #if GHC_STAGE < 2
          |
     |
5747 | #if GHC_STAGE < 2
     | ^
cc1.exe: all warnings being treated as errors

compiler\utils\FastString.hs:127:0: error:
     error: "GHC_STAGE" is not defined, evaluates to 0 [-Werror=undef]
      127 | #if GHC_STAGE >= 2
          |
    |
127 | #if GHC_STAGE >= 2
    | ^

compiler\utils\FastString.hs:336:0: error:
     error: "GHC_STAGE" is not defined, evaluates to 0 [-Werror=undef]
      336 | #if GHC_STAGE < 2
          |
    |
336 | #if GHC_STAGE < 2
    | ^
cc1.exe: all warnings being treated as errors

compiler\main\DynFlags.hs:1:1: error:
    `gcc.exe' failed in phase `C pre-processor'. (Exit code: 1)
  |
1 | {-# LANGUAGE CPP #-}
  | ^

compiler\utils\FastString.hs:1:1: error:
    `gcc.exe' failed in phase `C pre-processor'. (Exit code: 1)
  |
1 | -- (c) The University of Glasgow, 1997-2006
  | ^
)

I remember seeing this error a while ago but thought it had been fixed on HEAD.

I was able to do a quick build in the same tree.