How do I include calls to methods only present in one operating system version when compiling for multiple versions?

I have an app that is being compiled with a minimum system requirement of Tiger (OS X 10.4), but I need to include some code to deal with Spaces if the app is executing on Leopard (OS X 10.5).

How do I do this?

(The call I need to make is to -[NSWindow setCollectionBehavior:]).

1. Set your Base SDK (for all configurations of the target you're building) to the version of the OS that includes the call.
2. Set the Deployment Version to the earliest OS you want your code to launch on.
3. For all calls that don't exist in earlier versions of the OS, you must test before you call, either by using respondsToSelector: (for methods) or testing against nil (for functions and statics). You can if you like do a check for the OS version, but it's more robust to check the specific call.

The Cross-Development Programming Guide describes the techniques in detail.