The public interface of JavaScriptCore has no version of its own. The Mac port actually introduced WebKitAvailabillity.h some time ago to share the version number between JavaScriptCore and WebKit. The situation is becoming particularly interesting now since JavaScriptCore gains new API and ports other than Mac cannot check any kind of version number. For instance JSContextGroupRef was introduced 2008-07-29, "Implement JSContextGroup APIs to make concurrent execution possible for JavaScriptCore clients". The commit intorduced new API and changed behaviour. I don't know what ports apart from Gtk are actually shipping JavaScriptCore headers as part of their API right now, but a port agnostic version seems like the right solution here.
JavaScriptCore is now versioned along with WebKitGTK+. If we ever ship a version of JSC separate from WebKitGTK+, it will have it's own versioning.