I kind of thought so. I have a fair amount of heartburn on Mojave and its predecessors when having to compile. This happened the other day with an makefile with a by-now standard flag that Apple, for some reason, has decided not to expose.
Here's what I'd suggest to your Apple cohort:
- Upgrade to Mojave through the App Store, if they haven't already.
- From the App Store download the XCode App.
- From the terminal
$ xcode-select --install
The process can take up to a couple of hours, unfortunately, but should fix the Rcpp problem and many future compile problems.