Thanks !!
I applied the patch and then everything ran smoothly from there, and was able to go through
$ sudo port install git exiv2 libgphoto2 gtk-osx-application-gtk3 lensfun librsvg libsoup openexr json-glib flickcurl GraphicsMagick openjpeg lua webp libsecret pugixml osm-gps-map adwaita-icon-theme tango-icon-theme intltool iso-codes libomp
entirely. Indeed I see more recent commits on rawspeed dealing with openmp (like 369c170), and on DT it seems that the most recent commit in src/external/rawspeed is 5 days old (1ca1601).
The latter seems to be the one I need to consider.
Thanks again for helping 