> Apple Clang requires passing "-Xpreprocessor -fopenmp" rather than just -fopenmp.

Actually, either "-Xpreprocessor -fopenmp" or "-Wp,-fopenmp".

However, libtool (version 2.5.3) supports only "-Wp,-fopenmp". If you try compiling a library with CC="cc -Xpreprocessor -fopenmp", libtool will produce an error message:

clang: error: unsupported option '-fopenmp'

So, the only remaining possibility is to add try "-Wp,-fopenmp".

But programs that use OpenMP also expect an <omp.h> header file to be present. Neither Apple's clang nor the upstream LLVM/clang has such a (complete, usable) header file of this name so far.

So, my take on this is: No need to do anything for now. And when clang does have an <omp.h> file and a corresponding library, the AC_OPENMP macro should be extended not only to test whether the compiler supports "-Wp,-fopenmp" but also test whether "#include <omp.h>" works.