cpplib: Fix __cplusplus definition

[Benjamin Kosnik]

> | No joy.  :-(  I get the same results from three fresh builds, using each
> | of c, c_std, c_shadow.
> 
> Then, something must be wrong with our architecture -- we should coope
> with compliant system headers.  Sigh.

Hmm. I'm not quite sure we can do anything else. We cope with:

1) unmangled, compliant headers by doing nothing (c)

2) standard "C" headers by injecting into std:: (c_std)

3) an attempt at only std:: (c_shadow)


It looks like the error may be somewhere else, in the way the sun headers 
inject into std. 

Phil if this is the case you shouldn't be able to compile

#include <stdio.h>

with g++. Can you try that? if it doesn't work, can you pre-process and 
post it?

thanks,
benjamin