I see 20180718 in your build line, so I'm thinking you don't yet have the current portfile.
sudo port clean widelands-devel to clean out the remnants of the old build.
sudo port selfupdate to get the newest Portfiles from MacPorts.
Then you should see
$ port info widelands-devel widelands-devel @20190422 (games)
and then it should build.