The old binary/url is no longer available, and the layout of the
sourceforge archvie seems to have changed to include a new
old_versions/ directory where the previous binary is
relocated.
https://sourceforge.net/projects/winflexbison/files/old_versions/
While we could use that, it seem better to invest the effort to
adapt the scipt afterwards to use the github archives instead
https://github.com/lexxmark/winflexbison/releases
So for now bump the version to get the subproject building
till we switch to github releases.
9e7855866a