Thank you Andrea!
For documentation purposes, I downloaded / installed / reboot XQuartz 2.7.11 with no luck. My next step is to get rid of MRO 3.5....
ok. It's now working thanks to Andrea!
Here is what I did:
- Removed MRO 3.5 from my system
- Downloaded / installed XQuartz.2.7.11
- Downloaded / installed R 3.5 (cran.us.r-projet.org)
- Downloaded / installed clang-6.0.0.pkg (cran.us.r-projet.org)
- Downloaded / installed gfortran-6.1.pkg (cran.us.r-projet.org)
- Rebooted my system
Everything seems to work fine now!