Agree with @eht16 that we should merge this PR, and since I just installed a new Linux version and had to rebuild Geany, I also tested this PR and posted that it WFM, but then noticed 3.21.1 was released. So I suggest updating this to 3.21.1 if its to be the last "old compiler" release [Note: @eht16 pushed 3.21.1 while I was typing :) ].
There should be a release containing 3.21.1 so any old compiler users left have a defined last version and tarballs etc
@kugel- if Scintilla version 4 actually needs no changes to Geany then I agree that if a PR for it materialises after the release with Scintilla 3.21.1 but before stable Scintilla 5 (Neil has said some early releases of 5 will be unstable IIRC) is available then for sure Scintilla 4 can be included. I just didn't think it was a good use of our limited resources if Scintilla 4 needed work and then Scintilla 5 needed work.