Swift Package Manager cannot find header file

I have the following project structure:

├── Package.swift
├── Sources
│   └── Argon
│       ├── include
│       │   └── Argon.h
│       └── some_source.c
└── deps
    └── olive.c
        └── olive.c # Header file

Contents op Package.swift:

// swift-tools-version: 5.7
import PackageDescription

let package = Package(
  name: "MyPackage",
  targets: [
      name: "Argon",
      cSettings: [

Argon.h contains the follwing include:

#include <olive.c>

When I run swift build, I get the following error:

In file included from /Users/jonaseveraert/Documents/projects/photobooth/Sources/Argon/some_source.c:3:
/Users/jonaseveraert/Documents/projects/photobooth/Sources/Argon/src/../include/argon.h:41:10: fatal error: 'olive.c' file not found
#include <olive.c>
1 error generated.

It looks to me that the headerSearchPath is not working.

I have tried with .unsafeFlags(["-Idep/olive.c"]), but did not work.

I tried renaming olive.c to olive.h, did not work.

Anyone have any pointers on why this happens?

(yes, the header file ends with a .c extension, that's how the library creatorr named it)