[R-SIG-Mac] Xcode/CLT 11

[Prof Brian Ripley]

As a test (I would not recommend it), I installed Command Line Tools 11 
on my Mojave machine.

1) As expected, it does not contain any macOS_SDK_headers_for_macOS 
package, so the second approach in
https://cran.r-project.org/doc/manuals/r-devel/R-admin.html#Recommended-C_002fC_002b_002b-compilers
is needed to use the non-Apple builds of clang.  If you do not need 
OpenMP support, one can simple use the CLT's compilers.

2) This contains the SDK for 10.15 (as did CLT 10.2.1).  However, it 
conditionally declares timespec_get, a C11 function that is finally 
added in 10.15.  autoconf does not understand the conditioning, so R's 
configure defined HAVE_TIMESPEC_GET in src/include/config.h.  I have 
worked around this in R-patched and R-devel, but for earlier versions of 
R simply comment out that line in src/include/config.h after running 
configure and before make.

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