I had a similar question answered on the MacPorts mailing list[0].
Export these environment variables.
export CPPFLAGS='-isystem/opt/local/include'
export LDFLAGS='-L/opt/local/lib'
P.S. Hope you've not been waiting this long for an answer :)
[0] https://lists.macports.org/pipermail/macports-users/2017-July/043562.html