I think you're talking about "The SQLite Amalgamation" - https://www.sqlite.org/amalgamation.html. I hadn't heard of it before, so thanks for encouraging me to look it up - seems like it might come in handy!

I'm using a python script and a template to merge source files into ape.c so it's quite similar to sqlite's amalgamation. It's not as simple as joining a lot of files together though because there are dependencies between files and all global variables and functions need to be declared static.