cpplib: Fix __cplusplus definition

[Benjamin Kosnik]

> Please find pure.ii attached to this message.

This is a small example of what is going on for solaris 2.8 compiles:

//# 46 "/files/pfeifer/OBJ-0127-0049/gcc/include/sys/types.h" 2 3 4
#if 1
extern "C" {
typedef unsigned int size_t;
}
#endif

//# 31 "/usr/include/iso/time_iso.h" 2 3 4
extern "C" 
{
  namespace std 
  {
    typedef unsigned size_t;
  }
}


//# 21 "/usr/include/time.h" 2 3 4
using std::size_t;


It looks to me like what should be done is supress 
gcc/include/sys/types.h (size_t, ptrdiff_t)

-benjamin