it's fixed in SVN, try my built.

The cause is two fold (and has nothing to do with coded or built below El Capitan). SDL the backend of Dosbox and many other apps hasn't been fixed and when it concerns SDL 1.2x the SDL developers are reluctant to fix it *and* release a new version.
Dosbox shares it's audio code with Exult, ScummVM and a couple other ones and that part is outdated but produced a warning when building in earlier OS X versions as well.

I see. I guess that it's a coincidence for me then that I saw the error the first time when I upgraded to El Capitan (In one of my own games actually coded in BlitzMax, and considering how quick Apple is with deprecating and removing stuff, I guess I assumed things the wrong way).

And since console output is not shown in "normal" OS X usage, I wasn't sure if this was known since an issue like this could easily remain "under the hood".

Anyway I'm glad that a fix exists. Is it only an SVN now, or is this fix already available from the 'regular download page' as well?

Only in SVN, there hasn't been a new version for a while and it might take another while 😀

I uploaded the patch almost three years ago https://sourceforge.net/p/dosbox/patches/256/ (if you need a pointer with your own game) and the SDL patch is at https://bugzilla.libsdl.org/show_bug.cgi?id=3127 for both sDL 1.2x and SDL2.

Tricky wrote:

I see. I guess that it's a coincidence for me then that I saw the error

Just for the record, it's not an error. Just a deprecation warning. Which means they'll probably remove support for the old API years from now, so it's nothing to actually worry about.

Is there a chance DOSBox will be ported to SDL2? (yes, I know it is not trivial)

1.2 is more and more problematic.

An adaptation to SDL 2.0 (Alpha-level Android build attached)