Check clang<20 only for clang based compilers

Author: vinay-deshmukh

libcxx/include/__tree

@@ -628,7 +628,7 @@ public:
 // 2. AppleClang is not yet using clang >= 20.x, which has "fixed" this issue
 // 3. A few others like FreeBSD/amd64, AArch64, AIX
 // FIXME: when AppleClang is based off of clang >= 20.x
-#if _LIBCPP_CLANG_VER < 2000
+#if defined(_LIBCPP_CLANG_VER) && _LIBCPP_CLANG_VER < 2000
   _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX26 ~__tree_node() {}
 #else
   ~__tree_node()                             = delete;