Situation:

• We have been talking about dependencies a while recently, but never came to a decision.
• It looks like we like to introduce dependencies on fmtlib and ranges-v3 and maybe intx. There also was the idea to replace json-cpp with nlohmann/json.
• Our repo contains a copy of evmc.
• For json-cpp we use the cmake ExternalProject_Add mechanism
• We also depend on boost and optionally on Z3 and CVC4. For those we use plain standard cmake find_package.

Issues with ExternalProject_Add:

• There is no way to override it and dynamically link against system json-cpp instead.
• make requires a network connection and starts downloading
• The latter complicates e.g. PPA scripts.
• We can't pre-pack dependencies to our CI docker images, increasing build time.

Apart from boost, all the other dependencies except json-cpp (including nlohmann/json as potential json-cpp replacement) are header-only or nearly header-only.

When considering to add new dependencies we could:

• Reuse the ExternalProject_Add mechanism despite its drawbacks.
• Use git submodules for the new dependencies (or all dependencies).
• Instead of submodules use subtrees.
• Switch everything to plain standard cmake find_package, while maintaining a better ìnstall_deps script to locally install dependencies in the build tree.
• Combine find_package with other mechanisms as fallback.