Optionally allow the system installation of nlohmann_json to be used

[chewi]

Description

As before, this makes the Gentoo Linux package a little simpler. I did say that last time was my final unbundling request, but you've added this dependency since.

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Dependency update (updates to dependencies)
• Documentation update (changes to documentation)
• Repository update (changes to repository files, e.g. .github/...)

Checklist

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have added or updated the in code docstring/documentation-blocks for new or existing methods/components

Branch Updates

LizardByte requires that branches be up-to-date before merging. This means that after any PR is merged, this branch
must be updated before it can be merged. You must also
Allow edits from maintainers.

• I want maintainers to keep my branch updated

[ReenigneArcher]

Can we install it as a package in all of our build pipelines? That would be preferred to using a submodule anyway.

[chewi]

I was going to say no, as I didn't expect other distributions to bother, given that it's just a bunch of headers, but turns out they all have it!

• Arch - nlohmann-json
• Fedora - nlohmann-json-devel
• Debian/Ubuntu - nlohmann-json3-dev

[ReenigneArcher]

It's also available for Windows and macOS.

https://packages.msys2.org/base/mingw-w64-nlohmann-json
https://formulae.brew.sh/formula/nlohmann-json#default

Would you mind making a PR to use the installed package instead? It would require updates to our CI/Docker files, and docs. Otherwise, I can put it on my list of things to do.

[chewi]

I'm on it.

[chewi]

Closing in favour of #2161.