I've done something different before:

    // main.c
    #include <module.c>
    int main() {
        moduleFunction();
    }
    
    // module.c
    #include <stdio.h>
    void moduleFunction() {
        printf("calling modules");
    }

This has its issues (you can't do parallel compilation anymore; you don't have unexported functions, etc), but it works fine for small things. Super helpful when I want to make a small thing and don't want to bother with headers.

You just include all the C files in one file and be done with it.