SDK won't compile with Xcode 7.3.1

[boundsj]

Changes introduced in e2797d1 broke the project when using Xcode 7.3.1. Specifically:

• In MGLGeoJSONSource.h, const was moved so that MGLOfflinePackProgressChangedNotification, MGLOfflinePackErrorNotification, and MGLOfflinePackMaximumMapboxTilesReachedNotification are pointers to constant NSString instead of constant pointers to NSString. This fails to compile in Xcode 7.3.1 at the call site
• In MGLTypes
  • the new shim does not define NSErrorDomain and is not above where NSErrorDomain is used
  • #if !defined(FOUNDATION_SWIFT_SDK_EPOCH_LESS_THAN) || FOUNDATION_SWIFT_SDK_EPOCH_LESS_THAN(8) actually uses FOUNDATION_SWIFT_SDK_EPOCH_LESS_THAN which is not defined before iOS 10.0

cc @1ec5

[boundsj]

7.3.1 is required as noted in #6842

[1ec5]

#if !defined(FOUNDATION_SWIFT_SDK_EPOCH_LESS_THAN) || FOUNDATION_SWIFT_SDK_EPOCH_LESS_THAN(8) actually uses FOUNDATION_SWIFT_SDK_EPOCH_LESS_THAN which is not defined before iOS 10.0

Shouldn't it short-circuit on the first clause?

[boundsj]

Shouldn't it short-circuit on the first clause?

It did not appear to do that

[boundsj]

Fixed by #6923