Make additional IpcMessage get_value/set_value MacOS only.
Make additional IpcMessage get_value/set_value MacOS only.

Previous commit added additional get_value and set_value template
specialisations for unsigned long to fix clang linker errors on MacOS.
These cause multiple definition errors on Linux, so wrapped in
conditional ifdef blocks.
percival-detector/odin-data,percival-detector/odin-data,odin-detector/odin-data,percival-detector/odin-data,odin-detector/odin-data,odin-detector/odin-data,odin-detector/odin-data,percival-detector/odin-data,percival-detector/odin-data,odin-detector/odin-data,odin-detector/odin-data,odin-detector/odin-data,percival-detector/odin-data,percival-detector/odin-data