Fix static linking to Autotools builds using CMake

[smcv]

• autotools: Move mandatory dependencies from EXTRA_LDFLAGS to MIXER_LIBS

  Autotools variables whose names end with _LDFLAGS are usually for
  linker options such as -Wl,--export-dynamic which would not be
  appropriate to copy into the pkg-config or CMake metadata.

  Move these dependencies into a separate _LIBS variable, which is
  conventionally a list of -L and -l options (only) and can be copied
  into pkg-config or CMake metadata. This follows the same pattern as
  IMG_LIBS and TTF_LIBS in SDL_image and SDL_ttf.

  Exclude SDL_LIBS from this, because the CMake metadata has a special
  case for finding SDL, and linking to a different SDL causes link failure
  on our macOS CI. Link SDL via $(SDL_LIBS) instead.

• sdl2_mixer-config.cmake.in: Use MIXER_LIBS for static linking

  We can't rely on all mandatory dependencies being in PC_LIBS, because
  if we found a dependency such as libflac via pkg-config, we would have
  put the name of the pkg-config module in PC_REQUIRES instead.

  Resolves: Static linking of dependent program with CMake fails in Autotools build with --disable-foo-shared #411

[madebr]

Thanks!
The cmake config script now works with all dependencies being non-dynamic.