Depends on your needs.
If you don't need efficiency then the better alternative is to simply write your own evaluator. AVS script is so primitive that writing an evaluator (slow one) is almost trivial (parser might be little trickier).
If you want portability, then reusing AVS source code is not an option. The code is non-portable mess.
Only use AVS source if portability is not a concern and you really need the evaluator to be fast. But even then it might be easier to translate the script into some intermediate form (llvm, C--) and let the compiler do the optimization.
If you want to evaluate simple arithmetic expression... then really, write your own code. It should be well below 100 lines.
Anyways, you can reuse the AVS code quite freely, I think they use BSD 3 license.
Phi = (1+sqrt(5))/2