OpenKJ doesn't build against libfmt 9.1.0

[salfter]

The computer on which I run OpenKJ pulled in libfmt 9.1.0 in a recent systemwide upgrade. Once this happened, OpenKJ would no longer build:

build.log

I then downgraded libfmt:

sudo emerge -1 \<libfmt-9.1 && sudo emerge -1 openkj

This pulled in libfmt 8.1.1, against which OpenKJ (git master) would build. To confirm, I let libfmt upgrade again:

sudo emerge -1 libfmt && sudo emerge -1 openkj

This failed again. Something probably changed in a breaking way with libfmt.