[R-SIG-Mac] Warning on XCode / Command Line Tools 11

[Prof Brian Ripley]

My Mojave machine today prompted me to update to version 11 of Command 
Line Tools (apparently 'beta 1', but I am not a Beta tester).

In short: don't do that (you can revert to 10.2.1 from the URL above, at 
least if your Apple ID has (free) developer privileges).

CoreFoundation.framework has been removed (and its headers are used in a 
couple of spots compiling R).

Further its seems that 
/Library/Developer/CommandLineTools/Packages/macOS_SDK_headers_for_macOS_10.14.pkg 
is no longer included and setting CPPFLAGS did not suffice.

Hopefully we will have solutions in due course, but that version is 
really for 10.15 Catalina which is months away ('in the Fall', whenever 
that is).

-- 
Brian D. Ripley,                  ripley using stats.ox.ac.uk
Emeritus Professor of Applied Statistics, University of Oxford