[Bug]: Can't build on macOS: `'linux/futex.h' file not found`

### Describe the issue

The project fails to compile with `abseil` installed on my iOS project (that is a direct dependency of Protobuf-C++)
The build fail with the error:
`spinlock_linux.inc:17:10 'linux/futex.h' file not found`

I also tried to install abseil directly through [the pod](https://cocoapods.org/pods/abseil) but that's the same issue.

### Steps to reproduce the problem

- Create a basic iOS app.
- import Protobuf-C++ in the podfile (tweaked version to fix [an importing issue](https://github.com/protocolbuffers/protobuf/issues/13596) -> [used podspec fix here](https://github.com/protocolbuffers/protobuf/pull/16519))
- Import a header from protobuf that is using abseil

### What version of Abseil are you using?

tested on both:
- 1.20240116.1
- 1.20240116.2

### What operating system and version are you using?

macOS x64 (Ventura 13.6.5)

### What compiler and version are you using?

Apple clang version 15.0.0 (clang-1500.0.40.1)
Target: x86_64-apple-darwin22.6.0
Thread model: posix
InstalledDir: /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin

### What build system are you using?

I'm building through XCode.
C++17 is also used in the project settings.

### Additional context

Podfile:
```podfile
platform :ios, '12.4'

target 'MyProject' do
use_frameworks! :linkage => :static
pod 'Protobuf-C++', :git => 'https://github.com/takeh1k0/protobuf.git', :branch => 'feature/fix-protobuf-c++-podspec'
end
```