ERFC_SCALED compile-time simplification

[FX]

> If there are other Fortran-required functions not currently in MPFR,  
> it
> might be worth requesting them as well - implementing functions  
> required
> for Fortran seems just as reasonable a thing for MPFR to aim to do as
> implementing the C99 functions.

I think all other real-valued functions we need are in MPFR already.

By the way, I see that the debate around MPC integration seems to have  
died a bit. It'd like to say that it would be a really great progress  
for gfortran to have that in, as it would enable better standard  
conformance at a low maintainability price. I think Kaveh has  
summarized it well: mainly, it is guaranteed to build on all primary  
and secondary plaforms, and it can be built in-tree so it's not adding  
any dependency.

FX