Allow to use external deps?

[aldanor]

If a project already uses one of the deps (especially if it’s within the same cmake build), it would nice to be able to tell spdlog_setup to use them.

[guangie88]

My original focus was to make it simpler to get all external deps in one place, but missed out the fact that external deps should be searched first before looking into the git submodule deps.

Currently I don't really have the time to try it out, but that could be the next enhancement for it. Will be happy to have a PR for it if there is any. Thanks!