We currently disable deprecation synchronization warnings in case we're
building with Clang. We check for Clang by doing a string comparison on
the compiler identification, but this seems to have been broken by an
update in macOS' image as the compiler ID has changed to "AppleClang".
Let's just unconditionally disable this warning on Unix platforms. We
never add the deprecated attribute anyway, so the warning doesn't help
us at all.