[derefr]

> You can't do it in C unless you also implement a compiler or evaluator.

Someone else already did that work. Your C compiler is very likely, in fact, already written in C. We don't expect a C-importable library interface for the C compiler, because none of the traditional old C compilers had one (gcc, MSVC, etc.) But libclang exists, and it's pretty easy to write a C REPL using it. From there, only a few steps to automation.