Your advice to use CLP(FD) for the semantics of integer math operations was fantastic!
I typically use SWI Prolog but I hope the code is mostly portable. The parser uses a couple of DCGs from the "basics" lib but I think those are also portable or at least simple to re-implement.
The tricky bit might be the semantics of math (and comparison) ops. I've tried two other sets of semantics, one that attempts to perform math operations (and catch the errors if e.g. an arg is a logic var rather than an int) as you go, and another that just builds expression trees (evaluation is delayed) like so:
func(+, [int(A), int(B)|S], [int(A + B)|S]).