If your code requires a feature that is only available in macOS 10.12 or newer, but you want your code to also deploy to earlier system versions, you can use @available in Objective-C:

if (@available(macOS 10.12, *)) {
    // Code that requires 10.12 APIs
} else {
    // Code that doesn't require 10.12 APIs
}

You can do the same thing in Swift using #available:

if #available(macOS 10.12, *) {
    // Code that requires 10.12 APIs
} else {
    // Code that doesn't require 10.12 APIs
}

But what can you use if you write C or C++ code?