[Rd] Access to integer value of BUILTINSXP/SPECIALSXP

[Saptarshi Guha]

Hello,
For the BUILTINSXP and SPECIALSXP types, the R Internals page documents

"An integer giving the offset into the table of primitives/.Internals. "

What macro gives me the value of this integer? I guess something like  
this would work
had R_USEINTERNALS been defined
v->u.primsxp.offset
(where v is a SEXP of either of the above types).

What is the portable version?

Thank you
Saptarshi