Building dav1d in a vendored build fails for me on Fedora Linux

[nightmareci]

While I was trying to do a vendored build of SDL3 SDL_image on Fedora Linux, dav1d was failing to build, due to posix_memalign() not being defined, despite the build system setting HAVE_POSIX_MEMALIGN; it seems the check_symbol_exists(posix_memalign "stdlib.h" HAVE_POSIX_MEMALIGN) line is finding posix_memalign(), despite posix_memalign() breaking the build. Turns out, per the Linux documentation, you need to define _POSIX_C_SOURCE to 200112L or higher for posix_memalign(). I made the following change to dav1d's CMakeLists.txt, then dav1d built successfully:

# The WIN32 part is already in the CMakeLists.txt file, it's just here to hint where my change was made
# I'm not sure if checking only UNIX is correct, but it at least works for me on Fedora Linux
if(WIN32)
  add_compile_definitions(
    _WIN32_WINNT=0x0601
    UNICODE=1
    _UNICODE=1
    __USE_MINGW_ANSI_STDIO=1
    _CRT_DECLARE_NONSTDC_NAMES=1
  )
  check_symbol_exists(fseeko stdio.h STDIO_H_PROVIDES_FSEEKO)
  if(STDIO_H_PROVIDES_FSEEKO)
    add_compile_definitions(_FILE_OFFSET_BITS=64)
  else()
    add_compile_definitions(fseeko=_fseeki64 ftello=_ftelli64)
  endif()
elseif(UNIX)
  add_compile_definitions(
    _POSIX_C_SOURCE=200112L
  )
endif()

For whatever reason, dav1d successfully builds for SDL2 SDL_image, despite the SDL2 SDL_image's vendored dav1d CMakeLists.txt not defining _POSIX_C_SOURCE anywhere, maybe _POSIX_C_SOURCE is getting defined somewhere else. For the sake of strict correctness, it may be appropriate to define _POSIX_C_SOURCE for SDL2 SDL_image's vendored dav1d as well.