[R-SIG-Mac] Apple Clang does support OpenMP (if libomp is available)

[Jon Clayden]

Hi James,

> > Lack of OpenMP support in Apple’s build of Clang is cited as one reason for not using it in CRAN builds
>
> From R 3.4.x forward, OpenMP has been enabled in CRAN builds as the toolchain is using a custom compiler.

I’m aware of that, but this makes building packages from source more
complicated for users in my opinion, since they also have to install
the custom compiler toolchains. But whatever one’s views on that, I’m
suggesting that one reason given for not using Apple Clang may not be
entirely correct.

> > but this is only partly true: after installing libomp ***from Homebrew***, I have been adding
>
> Unfortunately, both Xcode and Xcode CLI do not ship clang with support for OpenMP. By installing `libomp` from homebrew, the system is getting a clang-compliant OpenMP compiler from the llvm formula. To check, please see:

Not at all. The libomp formula is standalone, with no dependency on
the llvm formula. See
https://github.com/Homebrew/homebrew-core/blob/master/Formula/libomp.rb.
I can assure you that I don’t have the llvm formula installed.

> which clang

/usr/bin/clang

All the best,
Jon