Bug#951087: Bug#951943: blockattack: FTBFS: SagoDataHolder.hpp:26:10: fatal error: SDL_mixer.h: No such file or directory

[Simon McVittie]

Control: tags 951943 + patch

On Sun, 23 Feb 2020 at 08:31:49 +0100, Lucas Nussbaum wrote (in #951087):
> During a rebuild of all packages in sid, your package failed to build
> on amd64.
...
> > /<<PKGBUILDDIR>>/source/code/sago/SagoDataHolder.hpp:26:10: fatal error: SDL_mixer.h: No such file or directory
> >    26 | #include <SDL_mixer.h>      //Used for sound & music

blockattack has a FindSDL2.cmake "find-module", but not the same one
as in megaglest, openjk, rbdoom3bfg and spring. The implementation in
blockattack happens to be unaffected by <https://bugs.debian.org/951087>.

However, blockattack assumes that the CFLAGS necessary to find SDL.h
are also sufficient to find SDL_mixer.h, similar to the neverball
issue mentioned in <https://bugs.debian.org/909740> and addressed in
<https://salsa.debian.org/sdl-team/libsdl2/commit/6f58f10282cf9b9af567ec520f0d2c4dc368dbea>.
The solution from 6f58f102 doesn't work here, because blockattack's
FindSDL2.cmake doesn't make use of pkg-config.

There are two solutions:

- fix blockattack so it doesn't make that assumption
  <https://salsa.debian.org/games-team/blockattack/merge_requests/1>;
- change SDL2 so it fits that assumption again, for example
  <https://salsa.debian.org/sdl-team/libsdl2/merge_requests/3>

I've confirmed that either of these solutions is sufficient, but for
best robustness I think we should do both.

Patches from blockattack!1 attached (successfully compiled but otherwise
untested).

blockattack should ideally also drop FindSDL2.cmake and rely on the
CMake support in SDL2 >= 2.0.4 (it is now a "config-file package" in CMake
terminology), similar to <https://github.com/JACoders/OpenJK/pull/1033>,
but that is not required to resolve this bug.

    smcv
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