One can also (ab)use the build system to run arbitrary preprocessing steps with any language over the "C" input. You can have recursive macros by using M4 or Perl or Python or some other language to expand them, converting your "foo.c.in" into a "foo.c" to hand off to the C preprocessor & compiler. It still feels dirty, but it's often much easier to understand & debug.
Yes, 100%. And since CPP doesn't actually understand C, it's not too hard to do some lightweight preprocessing that requires some real additional parsing.
But while CPP is pretty finicky and not very modern, getting such things working seamlessly with C build systems can be vastly worse (though better than the days where the GNU tools were ubiquitous).
I tend to find meson easy to use compared to all the others, and do this kind of thing, but it's still difficult and brittle.
