FMS on macOS with OpenMP

[climbfuji]

Building FMS on macOS with clang (Apple's native clang, not LLVM clang that can be installed via homebrew) and OpenMP enabled has always been a problem, because the OpenMP library for Apple's clang must be installed separately. In spack-we can solve this using

...
variant("openmp", default=True, description="Use OpenMP")
...
depends_on("llvm-openmp", when="+openmp %apple-clang", type=("build", "run"))

So far, so good. But when using such a (static, unfortunately; shared would be easy) FMS library in a cmake project, it seems that this dependency on llvm-openmp's libomp doesn't make it into the makefiles. There are ways to work around this, for example building the entire project that links to FMS with

-DCMAKE_SHARED_LINKER_FLAGS="/path/to/llvm-openmp/lib/libomp.dylib"

but that's far from ideal. It seems to me that FMS should be able to present itself correctly to cmake when find_package(OpenMP C Fortran) and target_link_library(...) is used, and not only link to the gfortran libgomp.dylib on macOS, but also to the C OpenMP library on macOS. Or am I wrong?

Unlike other packages, e.g. netCDF, there is no explicit way to link against a C OpenMP library:

target_link_libraries(fv3 PUBLIC NetCDF::NetCDF_C NetCDF::NetCDF_Fortran)

For OpenMP, it's only

target_link_libraries(fv3 PUBLIC OpenMP::OpenMP_Fortran)

and OpenMP::C does not exist/leads to an error.