Define BOOST_NO_CXX20_HDR_RANGES for libc++ < 18

At least for Clang the `-fexperimental-library` flag is required to
enable full range support.
Otherwise e.g. `std::ranges::join_view` won't be available.

include/boost/config/stdlib/libcpp.hpp

@@ -168,6 +168,11 @@
 #  define BOOST_NO_CXX14_HDR_SHARED_MUTEX
 #endif
 
+// libc++ < 18 requires experimental support enabled, e.g. `-fexperimental-library`
+#if (_LIBCPP_VERSION < 180000) && !defined(_LIBCPP_ENABLE_EXPERIMENTAL)
+#  define BOOST_NO_CXX20_HDR_RANGES
+#endif
+
 #if _LIBCPP_VERSION >= 15000
 //
 // Unary function is now deprecated in C++11 and later: