You don't have XCode and its command line tools installed. Go to the App Store and download the free XCode app and then, from the terminal enter
xcode-select -p
Even if you have XCode and the command line tools, it may fail anyway. This is because Apple has a non-standard compiler. If this is the case, in 7-10s, in normal times, St. Simon Urbanek of the R Core Team, will edit the source code and post a MAC binary. If you get a non-zero exit error the first time (it sometimes works all right), either wait or if you get the "binary ... but source is later" respond no.
It worked fine, I found the packages and they were operational in RStudio, but a little further on I bounced into this problem , thanks for the help on the first topic!