Cannot compile with `make` and `WERROR`

Working on HEAD with make, and setting WERROR=-Werror in my build.mk, compilation is stopped with

utils/hsc2hs/Compat/TempFile.hs:20:5: error:
     error: macro expansion producing 'defined' has undefined behavior [-Werror,-Wexpansion-to-defined]
   |
20 | #if NEEDS_TEMP_WORKAROUND
   |     ^
#if NEEDS_TEMP_WORKAROUND
    ^

utils/hsc2hs/Compat/TempFile.hs:18:35: error:
     note: expanded from macro 'NEEDS_TEMP_WORKAROUND'
                                   && defined(mingw32_HOST_OS))
                                      ^
   |
18 |                                && defined(mingw32_HOST_OS))
   |                                   ^

and similar errors. Disabling -Werror allows compilation to proceed.

I'm working on a Mac, v 10.13.6.

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