The picoc interpreter, created by Zik Saleeba, is an embeddable, minimal C interpreter environment that you can integrate into your system. On x86, the stripped executable ends up only 118kB, with the standard libraries, so it is very compact.
Browsing the code, the include module looks easy enough to work with, so you can extend the environment for your specific needs. The code is BSD licensed, allowing you many freedoms.
Looking at the code, it does suffer from some code rot. The last commit is a year old. However, the code looks well structured and builds nicely, so it is not very hard to maintain.