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

[Tom Tromey]

>>>>> "Rainer" == Rainer Orth <ro@CeBiTec.Uni-Bielefeld.DE> writes:

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

Rainer> with the prerequisite patches now installed, here's the reworked version
Rainer> of the final patch.

Rainer> Tested as described for the rest, ok for mainline?

Ok.

Tom