Fix compilation with older XCode 14.2

[gaborcsardi]

Apparently this is what CRAN uses. I can't install it on my macOS, I had to check it on Monterey, with tart.

On Monterey, I installed XCode 14.2 (c.f. https://gist.github.com/yamaya/2924292 and the output of the CRAN installation showing Apple clang version 14.0.0 (clang-1400.0.29.202)).

Download links are at https://xcodereleases.com/?scope=release but it is also OK to download only the command line tools after logging at the Apple Developer Portal.

[gaborcsardi]

Another way to test this is to put this in ~/.R/Makevars:

SDK=/Library/Developer/CommandLineTools/SDKs/MacOSX11.3.sdk
CC = clang -arch arm64 -isysroot $(SDK)
CXX = clang++ -arch arm64 -std=gnu++17 -isysroot $(SDK)
CXX11 = $CXX
CXX14 = $CXX
CXX17 = $CXX
CXX20 = $CXX
CXX23 = $CXX