Difficulty installing udunits2 package on ARM-based (M1) Macs

[cboettig]

Users report difficulty installing udunits2 on ARM-based Macs. note that binaries are not available on CRAN for this package (currently).

Installing binaries as directed from https://mac.r-project.org/libs-arm64/ and linking explicitly with:

 install.packages("udunits2", configure.args='--with-udunits2-lib=/opt/R/arm64/lib --with-udunits2-include=/opt/R/arm64/include')

seems to solve the problem.