Windows fixes

Some small fixes for extconf to correctly build gecode. In addition, I encountered some compilation errors I didn't see when we first tested windows builds. I compared this with an older copy of the source I had lying around and it seems that gecode have regenerated their flex/bison parser generator files with newer versions of flex and bison, without updating the version number. Importing the older versions of these works. Since these are used to build the "flatzinc" interpreter which we don't use, this should not affect any functionality used by dep-selector.