`:::` does not work in `Makevars.win`

```
MPI_ROOT = $(shell ${R_SCMD} \
                     "library(pbdMPI);get.conf('MPI_ROOT','"${R_ARCH}"','pbdMPI')")
```
is working, but not
```
MPI_ROOT = $(shell ${R_SCMD} \
                     "pbdMPI:::get.conf('MPI_ROOT','"${R_ARCH}"','pbdMPI')")
```