CoreBluetooth - fix compilation errors for 10.13 SDK

Apple broke source code compatibility by moving 'identifier' properties from
CBPeripheral and CBCentral classes into the new CBPeer class and marking it
as "since 10.13". As a result we have compilation errors in the code
that was valid since 10.7. To workaround this issue, we introduce a
category with 'identifier' property declarations for SDK >= 10.13
and target < 10.13.

Change-Id: Id7c9d33a9c8ae7627a06b4c1930e70cede1e5b07
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@qt.io>

src/bluetooth/osx/osxbtutility_p.h

@@ -313,4 +313,23 @@ Q_DECLARE_LOGGING_CATEGORY(QT_BT_OSX)
 
 QT_END_NAMESPACE
 
+#if QT_MACOS_PLATFORM_SDK_EQUAL_OR_ABOVE(101300) && QT_MACOS_DEPLOYMENT_TARGET_BELOW(101300)
+
+ // In the macOS 10.13 SDK, the identifier property was moved from the CBPeripheral
+ // and CBCentral classes to a new CBPeer base class. Because CBPeer is only available
+ // on macOS 10.13 and above, the same is true for -[CBPeer identifier]. However,
+ // since we know that the derived classes have always had this property,
+ // we'll explicitly mark its availability here. This will not adversely affect
+ // using the identifier through the CBPeer base class, which will still require macOS 10.13.
+
+@interface CBPeripheral (UnguardedWorkaround)
+@property (readonly, nonatomic) NSUUID *identifier NS_AVAILABLE(10_7, 5_0);
+@end
+
+@interface CBCentral (UnguardedWorkaround)
+@property (readonly, nonatomic) NSUUID *identifier NS_AVAILABLE(10_7, 5_0);
+@end
+
+#endif
+
 #endif