HOST_OS macros not set

See this repo: https://github.com/fintara/hls-bug-error-directive

[There are](https://github.com/fintara/hls-bug-error-directive/blob/trunk/app/Main.hs) a bunch of `#ifdef` for `_HOST_OS` and while the project compiles fine, HLS jumps to the `else` branch.

```
# error Unsupported platform
in /Users/fintara/hls-bug-error-directive/app/Main.hs at line 18 col 1
CallStack (from HasCallStack):
error, called at ./Language/Preprocessor/Cpphs/CppIfdef.hs:113:21 in
cpphs-1.20.9.1-11b55022:Language.Preprocessor.Cpphs.CppIfdef
```
(it can be seen by calling `haskell-language-server .` in the repo's directory)

I've tried to manually `#include ` but it doesn't seem to have any effect. (afaik this is the auto-generated header file which contains these macros).

This is with cabal 3.10.1.0, ghc 9.2.8, HLS 2.1.0.0.
The HLS is compiled with this command: `ghcup compile hls --version 2.1.0.0 --ghc 9.2.8 --cabal-update`
I'm using macOS 13.5, but the same is observable on ArchLinux.