Instead of the library that may be bundled with spdlog, because spdlog may be distributed without a bundled fmt (e.g. in Arch Linux's repositories).
This also changes both fmt and spdlog to be built as static libraries instead of used as header-only libraries, as that's the recommended approach.
This changes the archive's directory structure to use the standard layout on Linux. While it makes less sense on Windows, it's probably easier to deal with a consistent layout.
Managed using the new LIBLOOT_BUILD_TESTS config option, which defaults to ON.
Also manage the Google Test and testing-plugins dependencies using FetchContent, as it integrates more neatly with FindPackage.
This means that if you're not interested in the tests, you don't need to build them or download and build Google Test or download the testing plugins.
Oliver Hamlet