We are discovering, due to the Xcode 11 upgrade, just how many places in
MacPorts the path to the 10.14 SDK got baked into files where we didn't mean
for it to be. There are many other mailing list threads and tickets being filed
with us about this. Until this is all ironed out, your simplest course of
action would be to uninstall Xcode 11 and reinstall Xcode 10.
Will there be a notification or something when this is all ironed out?
Unlikely, as its a distributed issue, that needs fixes in numerous ports
etc. There probably will not be a single moment when 'everything gets
fixed' just a gradual fixing of the various issues over time.
I suggest instead you add yourself to the cc list in whatever tickets
are open for the ports in question you care about.
https://trac.macports.org/wiki/Tickets
Chris