The gnucash port at present has a single option to choose on which guile version to depend, which defaults to guile1.
Changing the option's default has proven problematic, due to guile1 and guile2 conflicting with each other.
At the same time guile1 is old and unsupported upstream, and the gnucash developers have stated they intend to drop guile1 support in the next major release.
Converting those options to flavors allows us to distribute both versions so also binary package users have an option. This also allows to change the default to guile2 since users can change flavor, if desired.
I've also added an UPDATING note to let the users know about this.