[R-pkg-devel] CXX14 not defined on Windows

I am testing building a package on Windows. The package requires C++14. 
I have installed the latest versions of R (3.6.2), RTools (3.5), and 
RStudio. The package builds fine under Linux.
I get the following error:

     > devtools::install_github("gdkrmr/zarr-R")
    ...
     ? C++14 standard requested but CXX14 is not defined
    ...


I have also set `~/.R/Makevars.win` as suggested in the manual "Writing 
R extensions", Section 1.2.5 "Using C++14 code". When typing `R CMD 
config CXX14` in the RStudio terminal, I get a reply 
("c:/Rtools/mingw_64/bin/g++"). When trying to install the package 
manually with `R CMD INSTALL ...` from the RStudio terminal, I get the 
same error.

What am I doing wrong?

Thanks!

Guido