I took the WIP patch and updated it for QGIS 3.12.1. I also added some
inputs for optional features and some Qt wrapping for the SVG icons to
show up correctly in the GUI.

Things that can still be improved:
 - Among the 550 tests, around 60 are failing. I disabled them for now
   in the package definition.
 - Also, I only put GPL2+ and GPL3+ as license (like in Gentoo and Nix),
   and not the complete list for every file (like in Debian).

Do you think this package definition for QGIS is good enough to be
merged in master (and improved later), or are the remaining issues
blocking?

P.S. This is related to issues #38150 and #38178 (patch to add GRASS).