Missing references to mpf_{to,from}_string

[CoffeeTableEspresso]

When compiling a simple example (ex1 but with one of the variables initialized using mpf_from_string), I get a linker error (undefined reference).

After grepping through the source code, I also did not find a definition of either of these two functions, just the declaration in tomfloat.h.

If I've screwed something up here and these two functions do exist, please let me know. Otherwise, I'd be more than willing to write a PR addressing this.