ENH: nagfor -  get_flags_linker_so() on darwin
ENH: nagfor -  get_flags_linker_so() on darwin

On Darwin, `nagfor` needs the same linker flags as `nag`.

This hopefully completes #19968.
mattip/numpy,rgommers/numpy,jakirkham/numpy,jakirkham/numpy,numpy/numpy,numpy/numpy,charris/numpy,mhvk/numpy,mattip/numpy,numpy/numpy,rgommers/numpy,endolith/numpy,jakirkham/numpy,mattip/numpy,mhvk/numpy,pdebuyl/numpy,anntzer/numpy,anntzer/numpy,mhvk/numpy,charris/numpy,mhvk/numpy,seberg/numpy,rgommers/numpy,numpy/numpy,pdebuyl/numpy,anntzer/numpy,seberg/numpy,seberg/numpy,mattip/numpy,anntzer/numpy,endolith/numpy,rgommers/numpy,charris/numpy,mhvk/numpy,pdebuyl/numpy,jakirkham/numpy,endolith/numpy,seberg/numpy,endolith/numpy,jakirkham/numpy,pdebuyl/numpy,charris/numpy