I could do #ifndef ARDUINO in all files, but I really do not like this approach.
OK - Now read your post and point out exactly where you asked any questions!Also , Why not, it's a perfectly sound and tested method.
Where are arrrgh and spdlog?
Unless the include was wrapped in a ifdef!
I meant to ask: where in the file system are those libraries?
Would it be possible for you to install those libraries somewhere outside of the src folder? That would prevent the source files from being automatically compiled and the solution of doing conditional #includes of those libraries would then work fine.
With the 1.5 Arduino library format (library.properties file in the library root folder, source files under the src subfolder), only the contents of the src folder are compiled.