Only add -Wno-unused-but-set-variable flag when using clang >= 13.0.0

The flag is only available for clang versions >= 13.0.0 according to the
manual for versions 12.0.1 and 13.0.0
- https://releases.llvm.org/12.0.1/tools/clang/docs/DiagnosticsReference.html
- https://releases.llvm.org/13.0.0/tools/clang/docs/DiagnosticsReference.html

mbedtls-sys/vendor/CMakeLists.txt

@@ -210,7 +210,10 @@ if(CMAKE_COMPILER_IS_GNU)
 endif(CMAKE_COMPILER_IS_GNU)
 
 if(CMAKE_COMPILER_IS_CLANG)
-    set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -Wall -Wno-unused-but-set-variable -Wextra -Wwrite-strings -Wpointer-arith -Wimplicit-fallthrough -Wshadow -Wvla -Wformat=2 -Wno-format-nonliteral")
+    set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -Wall -Wextra -Wwrite-strings -Wpointer-arith -Wimplicit-fallthrough -Wshadow -Wvla -Wformat=2 -Wno-format-nonliteral")
+    if(CMAKE_C_COMPILER_VERSION VERSION_GREATER 13.0.0)
+        set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -Wno-unused-but-set-variable")
+    endif()
     set(CMAKE_C_FLAGS_RELEASE     "-O2")
     set(CMAKE_C_FLAGS_DEBUG       "-O0 -g3")
     set(CMAKE_C_FLAGS_COVERAGE    "-O0 -g3 --coverage")