We map the Apple Clang versions to upstream, so that we have one
define to compare against.

Fixes build break on iOS due to qbasicatomic.h not defining
QT_BASIC_ATOMIC_HAS_CONSTRUCTORS on Apple Clang versions, which
is needed after 1e9db9f5



Change-Id: I17493c0187c20abc5d22e71944d62bfd16afbad2
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>