Without intending to step on any toes: I've got a git branch that builds on top of the changes of phab:D2613 and makes the jump straight to fmt 6.1.2 (which is the latest version at time of writing): https://github.com/s0600204/0ad/tree/fmt
Once D2613 is in, then if so desired I'll create a revision on Phabricator for consideration.
For linux: as fmt is available from linux distro repos, the build system now attempts to use that by default. This should work with any version of fmt from 4.0 upwards. However, for systems that don't have a recent enough version (see https://repology.org/project/fmt/versions), the minimum relevant source is bundled and can be compiled by passing --with-bundled-fmt to update-workspaces.sh.
For windows/osx: I don't have a system to test on, so some work there would be necessary. However, as said above, the relevant source is bundled, so it just be a case of tying into the respective build systems.
