After spending much of the day in a fruitless attempt to install OSX 10.11, only a small update for today:
Just changed the locations where saved games and the configuration get stored to something more standard-conform. For Linux, it's pretty clear what needs to go where, pretty much the same for OSX. I haven't tested latter, but the code is taken from v0.4, so should be okay. Windows turns out to be less easy to support properly, at least not without resorting to lots of Windows-specific code.
So this is another area where SDL2 might be of value, as it just happens to include a much better implementation for retrieving system directories than what I came up with.
The other part where it could be useful is in the switching between Window and Fullscreen mode. As I found out, SDL1.2 only support that on X11, and it doesn't do it well either (in testing I had cases where keyboard input no longer worked after switching. And killing the program while in fullscreen mode left X in a sad state).
And finally, with the scaling and screen size adjustments implemented in v0.4, the graphics quality should be much better than the blurry output you get right now, when the monitor does the actual scaling. (Though going full dynamic size would also require changes to the user interface. The alternative might be letterboxing)
So I am starting to warm for migrating to SDL2 after all. But first, achievements!