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

[Jon Clayden]

Dear all,

Lack of OpenMP support in Apple’s build of Clang is cited as one
reason for not using it in CRAN builds, but this is only partly true:
after installing libomp from Homebrew, I have been adding
“-Wp,-fopenmp” to CXXFLAGS and CFLAGS (and “-lomp” to LIBS) to my
builds for a while, and everything seems to work OK.

I’m not sure how far back this support goes (and I haven’t tried the
Xcode 11 tools yet), but is there any known reason not to take this
approach, and if not, would it make sense for R’s configure script to
try “-Wp,-fopenmp” when testing for OpenMP support?

All the best,
Jon