SDL3's CheckLibUnwind cmake macro autodetects libunwind for the
SDL3_test library and offers no option to disable it. On the buildbots
libunwind is present in staging, so the test programs end up linked
against it and packaging libsdl3-tests fails:
 Package libsdl3-tests is missing dependencies for the following libraries:
 libunwind-x86_64.so.8
 libunwind.so.8
This aborts the whole sdl3 build before anything is staged, taking
down every package depending on SDL3 or SDL2 (sdl2-compat, sdl2-image,
sdl2-mixer, sdl3-image, sdl3-mixer, sdl3-doom, gzdoom, vice).
Preseed the three detection cache variables to OFF so the result no
longer depends on whether libunwind happens to be staged; it is only
used for symbolised backtraces in test leak reports.
Fixes: 51549dadd5 ("sdl3: add SDL3 3.4.10")
Signed-off-by: Daniel Golle <daniel@makrotopia.org>