[elteto]

Because not everyone is using Makefiles or CMake.

A true header-only library should be build-system agnostic and this is one way to do that.

We can argue about build systems for C++ all day long and never come to an agreement. With this approach this piece of code can be used anywhere.

[pjmlp]

We can also argue C++ is not a scripting language, which is what is approach is all about.

When C and C++ were the main programming languages during the 1990's, and commercial compilers abounded strangely we could manage handling all those build systems approaches.

[elteto]

I think you are romanticizing the past. I’m pretty sure integration of third party C or C++ code sucked as much back then if not more. You just didn’t have as much of those dependencies because open source was in its infancy and the number of available libraries was much smaller too.