Thank you for linking that document: I was looking for it to no avail (briefly saw it when first trying to build, but later couldn't recall the source) when initially commenting on your packages, as it lends credence to my argument that there should be no UI toolkit dependencies. It was precisely upon seeing this document that my frustration with this package began, as it does the exact opposite of what upstream advises.
The purpose of Suil is to abstract plugin UI toolkits away from host code. To achieve this, Suil dynamically loads modules for the toolkits in use. The main Suil library does NOT depend on any toolkit libraries, and its package shouldn't either (otherwise, for example, every LV2 host in the distribution would depend directly on Gtk and Qt). Individual modules (like libsuil_x11_in_gtk3.so) should be packaged separately and themselves depend on the involved libraries or toolkits. These packages should also be versioned as described above to support parallel installation. Please do not make the main Suil package strongly depend on any toolkit package, this defeats the purpose of Suil and will cause problems for users who need to avoid a particular toolkit dependency for whatever reason. "Weak" or "recommended" dependencies are fine, the important thing is that users are able to avoid particular toolkits if they choose.
As you can see, suil's purpose is literally to not do what this script does; I hate fighting, but this seems so obvious to me that I am utterly befuddled by your decision.