Realpath not consistent between MacOs and Linux, so Branch on OS

[robertDurst]

To get the Makefile to work with my Mac, I looked at the output of the commands using realpath on my linux machine and translated them to what worked on the Mac. Need to do a bit of reading/testing to make sure the commands are Apples-to-Apples, but a basic smoke check is that things now actually compile on the Mac while just using make ✅