Use /std:clatest with MSVC, remove -Wunused-result

Use -std=gnu2x with Clang and GCC only. Use Use /std:clatest with MSVC.

Remove -Wunused-result. Clang and GCC enable it by default, and MSVC
doesn't support it.

CMakeLists.txt

@@ -324,9 +324,12 @@ if(AVIF_ENABLE_NODISCARD)
     else()
         unset(CMAKE_C_STANDARD)
         set_property(TARGET avif_obj PROPERTY C_STANDARD)
-        target_compile_options(avif_obj PUBLIC $<BUILD_INTERFACE:$<$<COMPILE_LANGUAGE:C>:-std=gnu2x>>)
+        if(CMAKE_C_COMPILER_ID MATCHES "Clang" OR CMAKE_C_COMPILER_ID MATCHES "GNU")
+            target_compile_options(avif_obj PUBLIC $<BUILD_INTERFACE:$<$<COMPILE_LANGUAGE:C>:-std=gnu2x>>)
+        elseif(CMAKE_C_COMPILER_ID MATCHES "MSVC")
+            target_compile_options(avif_obj PUBLIC $<BUILD_INTERFACE:$<$<COMPILE_LANGUAGE:C>:/std:clatest>>)
+        endif()
     endif()
-    target_compile_options(avif_obj PUBLIC $<BUILD_INTERFACE:-Wunused-result>)
     target_compile_definitions(avif_obj PUBLIC $<BUILD_INTERFACE:AVIF_ENABLE_NODISCARD=1>)
 endif()