Cannot build after switching to fmtlib

[kevinbackhouse]

Exiv2's OSS-Fuzz build is failing after we recently started using fmtlib. Our own workflows are able to build successfully on a wide variety of different platforms and compiler versions - the only thing that isn't working is OSS-Fuzz. On OSS-Fuzz, we're getting linker errors. Our best guess is that it's caused by the OSS-Fuzz's unusual configuration: Ubuntu 20.04, but with a much more recent clang version. Has anybody else seen anything similar and have any suggestions for how we can get fmtlib working on OSS-Fuzz?

For reference, this is the pull request where we started using fmtlib. It uses std::format when available, otherwise fmt::format, so we're able to build on multiple C++ versions, not just C++20 and up.