Fix for Xcode 13 RC targeting macOS on Big Sur (#19)

Fixes #18.

Author: stephencelis

Sources/CustomDump/Conformances/Foundation.swift

@@ -4,7 +4,8 @@ import Foundation
   import FoundationNetworking
 #endif
 
-#if compiler(>=5.5)
+// NB: Xcode 13 does not include macOS 12 SDK
+#if compiler(>=5.5) && !os(macOS)
   @available(iOS 15, macOS 12, tvOS 15, watchOS 8, *)
   extension AttributedString: CustomDumpRepresentable {
     public var customDumpValue: Any {

Sources/CustomDump/Conformances/UserNotifications.swift

@@ -105,7 +105,8 @@
     }
   }
 
-  #if compiler(>=5.5)
+  // NB: Xcode 13 does not include macOS 12 SDK
+  #if compiler(>=5.5) && !os(macOS)
     @available(iOS 15, macOS 12, tvOS 15, watchOS 8, *)
     extension UNNotificationInterruptionLevel: CustomDumpStringConvertible {
       public var customDumpDescription: String {

Tests/CustomDumpTests/DumpTests.swift

@@ -718,7 +718,7 @@ final class DumpTests: XCTestCase {
   func testFoundation() {
     var dump = ""
 
-    #if compiler(>=5.5)
+    #if compiler(>=5.5) && !os(macOS)
       if #available(iOS 15, macOS 12, tvOS 15, watchOS 8, *) {
         dump = ""
         customDump(