chibicc is great and also a very useful tool to do different kinds of C source code analysis and processing, such as https://github.com/rochus-keller/c2obx/
That looks like a very useful C library, thanks for the hint; as it seems it is much more than just a standard C library in that there are also threads, unicode and a lot (all?) of posix features; I do not have the full grasp yet, but it seems that I could use it as a runtime library for my Oberon+ language (which has a built-in FFI and can also transpile to C).