port incorrectly thinks Xcode command line tools are not installed

[manulcaux]

When trying to upgrade my ports, some of them fail with the following message :

Warning: The macOS 10.14 SDK does not appear to be installed. Ports may not build correctly.
Warning: You can install it as part of the Xcode Command Line Tools package by running `xcode-select --install'.

For example, ds9 won't build :

Error: Failed to build ds9: command execution failed
Error: See /opt/local/var/macports/logs/_opt_local_var_macports_sources_rsync.macports.org_release_tarballs_ports_science_ds9/ds9/main.log for details.

If I try to install the Xcode-tools, they are installed :

[Emmanuels:/Applications/cassis5.1last] caux% xcode-select --install
xcode-select: error: command line tools are already installed, use "Software Update" to install updates
[Emmanuels:/Applications/cassis5.1last] caux% 

How should I proceed ?

I am running MacOs 10.14.6 and Xcode 11.3