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This is a new variable that can be used to specify that a
given module relies on C++ exceptions or RTTI.

This is done for several reasons:

- First, we want to avoid always linking GNU libsupc++
  to every final binary, because we're soon going to
  provide an alternative.

- Second, it is useful to declare prebuilt binaries that
  have been compiled with these features. It allows the
  final link to work correctly.

Note that the change is backwards-compatible. You can still
use -frtti or -fexceptions in your compiler flags to enable
the features too.

Change-Id: I24b28935e4446c55b169d35990ec2a379ac08500