Apple GCC has extensions to support for both label attributes and
an "unavailable" attribute. These are critical for objc but are
also useful in regular C/C++.
Apparently at least the label attributes might have found their way to
upstream GCC but the code doesn't seem available on the GPLv2 tree so
we are taking the code directly from Apple. To make this clearer we
are preserving the annoying "APPLE LOCAL" tags and the ChangeLogs
when they are available.
Obtained from: Apple GCC 4.2 - 5531
MFC after: 3 weeks
2d278bb89e