Static linking of dependent program with CMake fails in Autotools build with --disable-foo-shared

[smcv]

To reproduce:

• Have a version of libflac that installs a pkg-config file
• Build SDL_mixer the way we do in Debian:
  • Autotools build
  • --enable-music-flac-libflac
  • --disable-music-flac-libflac-shared so that we rely on traditional static or dynamic linking rather than dlopen()
• Try to build the project in cmake/test/ with -DTEST_STATIC=ON (the default)

(The same problem probably exists for dependencies other than libflac, I haven't tried.)

Expected result: main_shared is linked statically, as far as is possible.

Actual result:

[3/4] Linking C executable main_static
FAILED: main_static 
: && /usr/bin/cc   CMakeFiles/main_static.dir/main.c.o -o main_static  /usr/lib/x86_64-linux-gnu/libSDL2.a  /usr/lib/x86_64-linux-gnu/libSDL2_mixer.a  -lm -ldl  -lasound -lm -ldl -lpthread -lpulse-simple -pthread -lpulse -pthread  -lX11 -lXext -lXcursor -lXinerama -lXi -lXfixes -lXrandr -lXss -lXxf86vm -ldrm -lgbm -lwayland-egl -lwayland-client -lwayland-cursor -lxkbcommon -ldecor-0 -lpthread -lrt && :
/usr/bin/ld: /usr/lib/x86_64-linux-gnu/libSDL2_mixer.a(music_flac.o): undefined reference to symbol 'FLAC__stream_decoder_get_total_samples'
/usr/bin/ld: /usr/lib/x86_64-linux-gnu/libFLAC.so.8: error adding symbols: DSO missing from command line
collect2: error: ld returned 1 exit status
[4/4] Linking C executable main_shared

I think this is because configure.ac has two code paths for mandatory dependencies:

• if found via pkg-config: add to PC_REQUIRES, which ends up in SDL2_mixer.pc Requires.private but is not mentioned in sdl2_mixer-config.cmake
• else: add to PC_LIBS, which ends up in SDL2_mixer.pc Libs.private and in sdl2_mixer-config.cmake _sdl2mixer_extra_static_libraries

SDL_image and SDL_ttf do not have this bug: they add mandatory dependencies to IMG_LIBS or TTF_LIBS as appropriate, and sdl2_foo-config.cmake lists both PC_LIBS and IMG_LIBS or TTF_LIBS in its extra_static_libraries variable.