If pdflatex is not installed, it is impossible to generate vignettes

Note this excerpt from devtools:::build

```
if (!vignettes) {
    args <- c(args, "--no-vignettes")
} else if (!nzchar(Sys.which("pdflatex"))) {
    message("pdflatex not found. Not building PDF vignettes.")
    args <- c(args, "--no-vignettes")
}
```
