Dealing with implementation differences

Hello all, How does one deal cleanly with implementation differences, e.g. things like isAscii being defined in Hugs but not in GHC? I'd hate to resort to Makefile trickery and preprocessing. Btw, the reason I want to run the same sources with Hugs and GHC is that I have a compiler whose parser is generated with Happy. Compiling the parser on a smallish computer takes ages and I don't need the efficiency at this stage anyway. On the other hand, I find the error messages given by GHC very good. If someone can suggest another compiler to try out or a way to reduce the time needed for compiling the parser, I'd be grateful. The parser is about 3500 lines of Haskell. -- Pertti