[Rd] R_ext/Altrep.h should be more C++-friendly

[Michael Sannella]

I am not able to #include "R_ext/Altrep.h" from a C++ file.  I think
it needs two changes:

1. add the same __cplusplus check as most of the other header files:
    #ifdef  __cplusplus
    extern "C" {
    #endif
        ...
    #ifdef  __cplusplus
    }
    #endif

2. change the line
    R_new_altrep(R_altrep_class_t class, SEXP data1, SEXP data2);
 to
    R_new_altrep(R_altrep_class_t cls, SEXP data1, SEXP data2);
 since C++ doesn't like an argument named 'class'

  ~~ Michael Sannella

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