There isn't a simple way to do this in C but I think muParser may be useful to you, it is written in C++ but has C binding. ExprTk is also an option but looks like it is C++ only, on the plus side it looks much easier to get interesting results with.
Another option may be the Expression Evaluation which is part of Libav. It is in C and the eval.h header has some good descriptions of the interface.