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, libjingle does not detect that and makes OVERRIDE a no-op.

This CL updates base/common.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=fischman@webrtc.org, wu@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/5159004

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

git-svn-id: a556c8bde68cedbc0208a1ffc78b06e9cd07f676@5255 4adac7df-926f-26a2-2b94-8c16560cd09d

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