Parse error when building module on FreeBSD

[FilWisher]

When building thyme on FreeBSD using stack, I get the following error:

Configuring thyme-0.3.5.5...
Building thyme-0.3.5.5...
Preprocessing library thyme-0.3.5.5...
[ 1 of 20] Compiling Data.Thyme.Format.Internal ( src/Data/Thyme/Format/Internal.hs, .stack-work/dist/x86_64-freebsd/Cabal-1.24.2.0/build/Data/Thyme/Format/Internal.o )
[ 2 of 20] Compiling Data.Thyme.Internal.Micro ( src/Data/Thyme/Internal/Micro.hs, .stack-work/dist/x86_64-freebsd/Cabal-1.24.2.0/build/Data/Thyme/Internal/Micro.o )
[ 3 of 20] Compiling Control.Lens     ( lens/Control/Lens.hs, .stack-work/dist/x86_64-freebsd/Cabal-1.24.2.0/build/Control/Lens.o )
[ 4 of 20] Compiling Data.Thyme.Calendar.Internal ( src/Data/Thyme/Calendar/Internal.hs, .stack-work/dist/x86_64-freebsd/Cabal-1.24.2.0/build/Data/Thyme/Calendar/Internal.o )

src/Data/Thyme/Calendar/Internal.hs:21:1: warning: [-Wunused-imports]
    The import of `Control.Applicative' is redundant
      except perhaps to import instances from `Control.Applicative'
    To import instances alone, use: import Control.Applicative()
[ 5 of 20] Compiling Data.Thyme.Calendar.MonthDay ( src/Data/Thyme/Calendar/MonthDay.hs, .stack-work/dist/x86_64-freebsd/Cabal-1.24.2.0/build/Data/Thyme/Calendar/MonthDay.o )

src/Data/Thyme/Calendar/MonthDay.hs:17:12: error:
    parse error on input `_'

Platform details:

FreeBSD 11.1-RELEASE amd64 GENERIC

[CGenie]

I can confirm the same error on FreeBSD 12.1-RELEASE r354233 GENERIC amd64

ghc --version
The Glorious Glasgow Haskell Compilation System, version 8.6.5

[tmcgilchrist]

The CPP macros are not being applied correctly and you end up with that underscore error.
Could you try on FreeBSD with fumieval#2 I don't have access to a FreeBSD machine right now but I believe it should fix the issue.