How to fix "configure: error: libproj not found in standard or given locations." when installing {{sf}} on macOS?

The error means you do not have the PROJ.4 library installed; it is the backend that handles various projection tasks (as you no doubt have guessed). {sf} requires three such backends - GDAL, PROJ and GEOS - to function correctly.

I am not a Mac user, but according to the documentation it might be handled by

brew install proj

on the command line.