Primarily relevant to JSON for Modern C++, but could be used for Eigen also.

Rather than having a complete duplicate of the header contents of an external software project, it would be preferable to instead have a set of software dependencies configured as git submodules, that would be pulled in at configure / build time and appropriately linked against. Incrementing the version of that dependency would then only involve creating a commit that updates the committish for that submodule. And it may be easier for developers of such softwares to track use of their software as submodules than it is to track duplications of their headers / instances of code that requires theirs to compile.