So in that case, should we assume that developers are expected to rely on the external packaging script for a proper build?
To build a full package, yes. To test the build, they can run the commands that you had in the makefile, which can be documented on the wiki. For instance, I have a script that builds various configuration before merging MRs (in fact I used to test Android too, until it got broken). In that setup I would download gradle etc. once, and setup the environment to build Wine with it. I wouldn't want to have to download gradle on every build, nor to have to store it inside the Wine tree.
Alternatively, we could probe for a suitable Gradle/AGP setup and fail early with a clear and concise error message if it’s missing, instead of attempting a build that cannot succeed.
Sure. -- https://gitlab.winehq.org/wine/wine/-/merge_requests/10354#note_133413