All these changes were upstreamed after 2.6.1
--- CMakeLists.txt.orig	2022-07-12 20:19:55 UTC
+++ CMakeLists.txt
@@ -294,14 +294,14 @@ if(SDL2MIXER_CMD)
     target_compile_definitions(SDL2_mixer PRIVATE MUSIC_CMD)
     set(fork_found OFF)
     if(NOT fork_found)
-        check_symbol_exists(fork sys/unistd.h HAVE_FORK)
+        check_symbol_exists(fork unistd.h HAVE_FORK)
         if(HAVE_FORK)
             set(fork_found ON)
             target_compile_definitions(SDL2_mixer PRIVATE HAVE_FORK)
         endif()
     endif()
     if(NOT fork_found)
-        check_symbol_exists(fork sys/unistd.h HAVE_VFORK)
+        check_symbol_exists(fork unistd.h HAVE_VFORK)
         if(HAVE_VFORK)
             set(fork_found ON)
             target_compile_definitions(SDL2_mixer PRIVATE HAVE_VFORK)
@@ -528,7 +528,7 @@ if(SDL2MIXER_FLAC_LIBFLAC)
             add_dependencies(SDL2_mixer FLAC)
         endif()
     else()
-        target_link_libraries(SDL2_mixer PRIVATE FLAC)
+        target_link_libraries(SDL2_mixer PRIVATE FLAC::FLAC)
     endif()
 endif()