[Rd] Add a new environment variable switch for the 'large version' check

[Dirk Eddelbuettel]

Or you use a fourth component to signal a development version as Rcpp has
done for years (and, IIRC, for longer than devtools et al used '9000').

There is no functional difference between 1.2.3.1 and 1.2.3.9000. They are
both larger than 1.2.3 (in the package_version() sense) and signal an
intermediate version between 1.2.3 and 1.2.4.

But one requires a patch. ¯\_(ツ)_/¯.

Dirk

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