- udunits2 is installed with MacPorts
$ curl -O https://mran.microsoft.com/snapshot/2018-08-01/src/contrib/units_0.6-0.tar.gz
$ sudo R CMD INSTALL units_0.6-0.tar.gz --configure-args='--with-udunits2-include=/opt/local/include/udunits2 --with-udunits2-lib=/opt/local/lib'