There are numerous small Lisp/Scheme implementations, and probably most of them are unfinished or unmaintained. I found serious, production-ready tiny Scheme interpreters for you to embed in your C or C++ programs.
I have an unhealthy obsession with one-file implementations of useful things in C, which you can just drop into your project and use without configuring libraries with dozens of files. Miniz is one such project: it is a single 225 KB
miniz.c, which contains a
zlib replacement plus functions to read/write ZIP files.
Linenoise is a small self-contained line editing library written in C by Salvatore Sanfilippo of Redis fame, aimed at replacing the monster readline and libedit libraries. It is used in Redis, MongoDB, and Android.