[Bug]: Error (out) for old MacOS

### Describe the issue

Abseil already [points](https://github.com/abseil/abseil-cpp#support) to google's "foundational C++ support policy" explicitly, but it still [carries](https://github.com/abseil/abseil-cpp/blob/20230802.0/absl/base/config.h#L525) code for old macos versions.

However, it seems this is not being tested anymore, as I found out when running against an older MacOS SDK:
```
In file included from $PREFIX/include/absl/synchronization/mutex.h:74:
$PREFIX/include/absl/synchronization/internal/kernel_timeout.h:94:45: error: unknown type name 'clockid_t'; did you mean 'clock_t'?
struct timespec MakeClockAbsoluteTimespec(clockid_t c) const;
^
/Applications/Xcode_13.2.1.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.9.sdk/usr/include/sys/_types/_clock_t.h:30:33: note: 'clock_t' declared here
typedef __darwin_clock_t clock_t;
```

It would be good to error out already in CMake based on what is being tested as the lower bound in CI

### Steps to reproduce the problem

Compile a project that includes `include/absl/synchronization/mutex.h` on an old MacOS

### What version of Abseil are you using?

20230802.0

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

MacOS

### What compiler and version are you using?

clang 15

### What build system are you using?

cmake

### Additional context

I'm not asking to support old MacOS, just error out more clearly if it's an unsupported version.