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

[Jason Merrill]

On 08/09/2011 09:14 AM, Marc Glisse wrote:
> I don't think we should define the C++ 2011 value yet. In my opinion, we
> should wait until:
> 1) the standard is official
> 2) gcc implements most of it: people will want to use __cplusplus as a
> test to know if they can use C++0X features, not if the compiler does
> some effort to implement half of them.

I'm of two minds about this, but I see that clang and edg still use 
199711L in C++0x mode, so let's stick with that for now.

Jason