SDL_SetHint not available with all supported SDL/SDL_mixer versions
Note: In particular, this affects anyone trying to build Freeciv 3.0.x on Ubuntu 18.04 LTS "Bionic Beaver": While SDL2 packages are available, the build dependencies of the Freeciv source package (apt build-dep freeciv) only include SDL 1.2 packages.
Build failure (in certain environments) introduced in #43549, on S3_0 branch only (since later branches don't support SDL < 2.0.0). SDL_SetHint is apparently only available since SDL 2.0.0, so building with only libsdl-mixer1.2 passes configure, but fails when compiling client/audio_sdl.c.