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

[Jason Merrill]

On 10/21/2011 06:22 PM, Mike Stump wrote:
> Is there a plan on what the default language will be?  C is stuck in 1980, and will be forever.  Personally, I'd rather have g++ move up language standards by default, I'd rather make the move before 2023.

I think it makes sense to change the default once C++11 support is 
mature, I'd guess in 4.9.

Jason