Build fix for Swift 5.9

Author: kateinoigakukun

Sources/JavaScriptKit/FundamentalObjects/JSObject.swift

@@ -206,12 +206,14 @@ public class JSObject: Equatable {
     /// This allows access to the global properties and global names by accessing the `JSObject` returned.
     public static var global: JSObject { return _global }
 
-    // `JSObject` storage itself is immutable, and use of `JSObject.global` from other
-    // threads maintains the same semantics as `globalThis` in JavaScript.
+    #if compiler(>=5.10)
     @LazyThreadLocal(initialize: {
         return JSObject(id: _JS_Predef_Value_Global)
     })
     private static var _global: JSObject
+    #else
+    private static let _global = JSObject(id: _JS_Predef_Value_Global)
+    #endif
 
     deinit {
         assertOnOwnerThread(hint: "deinitializing")