Closed issue by ericonr on void-packages repository

https://github.com/void-linux/void-packages/issues/18290

Description:
The [template](https://github.com/void-linux/void-packages/blob/master/srcpkgs/gtkmm/template) for gtkmm is using the same `short_dec` as gtkmm2, which specifies the version to be `(v2)`. In the case of gtkmm, it should be `(v3)`.