FWIW it's possible to amalgamate almost any C program automatically with the help of https://github.com/goblint/cil (a maintained fork of CIL): https://goblint.github.io/cil/merger.html

All you have to do is to make your CC this:

cilly --noPrintLn --merge --keepmerged

And in the end after compilation there will be a file named yourproject_comb.c