Don't link with atomic if built with apple clang

[iomaganaris]

See issue below:

: && /usr/bin/clang++ -O3 -DNDEBUG -arch arm64 -isysroot /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk -dynamiclib -Wl,-headerpad_max_install_names -o lib/liboomph_mpi.dylib -install_name @rpath/liboomph_mpi.dylib ext/oomph/CMakeFiles/oomph_mpi.dir/src/message_buffer.cpp.o
      ext/oomph/CMakeFiles/oomph_mpi.dir/src/communicator.cpp.o ext/oomph/CMakeFiles/oomph_mpi.dir/src/context.cpp.o ext/oomph/CMakeFiles/oomph_mpi.dir/src/request.cpp.o ext/oomph/CMakeFiles/oomph_mpi.dir/src/communicator_set.cpp.o ext/oomph/CMakeFiles/oomph_mpi.dir/src/communicator_state.cpp.o
      ext/oomph/CMakeFiles/oomph_mpi.dir/src/barrier.cpp.o ext/oomph/CMakeFiles/oomph_mpi.dir/src/mpi/context.cpp.o  -Wl,-rpath,/var/folders/5x/5gbpjy215c1_x31rmwc75t7m0000gp/T/tmp450kwt1j/build/lib  lib/liboomph_common.a  -latomic  lib/libhwmalloc.dylib  /opt/homebrew/Cellar/open-mpi/5.0.7/lib/libmpi.dylib && :
      ld: library 'atomic' not found
      clang++: error: linker command failed with exit code 1 (use -v to see invocation)

[msimberg]

cscs-ci run