[libcpp] Correctly define __cplusplus (PR libstdc++-v3/1773)

[Jason Merrill]

On 10/21/2011 03:11 PM, Marc Glisse wrote:
> Note that at least clang now defines __cplusplus to its new C++11 value
> (in experimental C++0X mode only). Apparently they switched around last
> June and say they are not the only ones. So if you want to follow their
> lead...

Hmm, between that and the fact that 4.7 will in fact have almost all of 
the C++11 features, I think changing the value makes sense.

Jason