Due to gnutls updated behaviour 
parafin changed the build description.
You need to perform (copies from BUILD.txt)
mkdir -p ~/ports/devel/gnutls/files
cp -R “$(port dir gnutls)” ~/ports/devel
curl -Lo ~/ports/devel/gnutls/files/patch.diff https://raw.github.com/darktable-org/darktable/master/packaging/macosx/gnutls-disable-connectx.diff
then append this line:
patchfiles-append patch.diff
to ~/ports/devel/gnutls/Portfile files you just copied (except for pugixml) and run:
$ portindex ~/ports
and then wait for a rawspeed patch for openmp support, that is already implemented in the rawspeed project but not yet transfered to darktables submodule directory - or copy the changed files from rawspeed project yourself.