https://github.com/abseil/abseil-cpp/issues/1465 reports that
some CMake builds on Apply platforms issue

```
warning: /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/ranlib: archive library: libabsl_bad_any_cast_impl.a the table of contents is empty (no object file members in the library define global symbols)
```

Our CMake build handles this problem for header-only libraries by not
building a library at all. For some libraries, for example our polyfills,
the library is only conditionally empty. In these libraries, I added
a single char variable on Apple platforms as a workaround.

I have been able to reproduce the warnings reported in
https://github.com/abseil/abseil-cpp/issues/1465, but they don't fail the
build for me. I don't see them any more after this change.

PiperOrigin-RevId: 595480705
Change-Id: Ie48637e84ebae2f2aea4e2de83b146f30f6a76b9