[R] Avoid re-building a vignette

[Terry Therneau]

I once knew the answer to this, but can't find where I found it.

Say I have a vignette that I don't want R to automatically rebuild, perhaps it uses some 
data I can't share or takes too long to run or whatever.  Say it is 
mypackage/vignettes/charlie.Rnw

It can create the pdf from an R session that actually has the data, place that result in 
inst/doc/charlie.pdf, then run
      R CMD build --no-vignettes mypackage

So far so good.  But then when I do R CMD INSTALL mypackage.tar.gz the intall process 
tries to run Sweave, resulting in a failure.   How do I get it to skip the Sweave call?

Terry Therneau