[ios, macos] Fixed Xcode 7.x compilation

Define NSErrorDomain before its first use. Define NSNotificationName as a typedef instead of a #define. Guard these shims on the existence of NS_STRING_ENUM rather than another macro that’s new to Xcode 8.

Fixes #6906.

platform/darwin/src/MGLTypes.h

@@ -20,6 +20,13 @@
 
 NS_ASSUME_NONNULL_BEGIN
 
+#ifndef NS_STRING_ENUM
+    #define NS_STRING_ENUM
+    #define NS_EXTENSIBLE_STRING_ENUM
+    typedef NSString *NSErrorDomain;
+    typedef NSString *NSNotificationName;
+#endif
+
 /** Indicates an error occurred in the Mapbox SDK. */
 extern NSErrorDomain const MGLErrorDomain;
 
@@ -97,9 +104,3 @@ NS_ASSUME_NONNULL_END
         #define NS_MUTABLE_DICTIONARY_OF(ObjectClass...)    NSMutableDictionary
     #endif
 #endif
-
-#if !defined(FOUNDATION_SWIFT_SDK_EPOCH_LESS_THAN) || FOUNDATION_SWIFT_SDK_EPOCH_LESS_THAN(8)
-    #define NS_STRING_ENUM
-    #define NS_EXTENSIBLE_STRING_ENUM
-    #define NSNotificationName NSString *
-#endif