Correctly define OVERRIDE when building with g++ 4.7 and C++11 support
Correctly define OVERRIDE when building with g++ 4.7 and C++11 support

g++ 4.7 and later support explicit virtual overrides when building with C++11
support enabled. However, webrtc does not detect that and makes OVERRIDE a
no-op.

This CL updates typedefs.h to define OVERRIDE properly when g++ 4.7 is used
with C++11 support enabled.

See this page for GCC support of C++11 features:
http://gcc.gnu.org/projects/cxx0x.html

R=andrew@webrtc.org, fischman@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/5149005

Patch from Chris Dumez <ch.dumez@samsung.com>.

Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 762fcdcca91860b4e69b6d4430851cc5e8197521

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