tentative fixes for clang failures.

include/boost/config/compiler/clang.hpp

@@ -257,8 +257,10 @@
 #endif
 
 #if !__has_feature(__cxx_decltype_auto__)
+#if (__clang_major__ > 3)
 #  define BOOST_NO_CXX14_DECLTYPE_AUTO
 #endif
+#endif
 
 #if !__has_feature(__cxx_aggregate_nsdmi__)
 #  define BOOST_NO_CXX14_AGGREGATE_NSDMI

include/boost/config/stdlib/libcpp.hpp

@@ -106,7 +106,7 @@
 #define BOOST_NO_CXX17_ITERATOR_TRAITS
 #define BOOST_NO_CXX17_STD_INVOKE      // Invoke support is incomplete (no invoke_result)
 
-#if (_LIBCPP_VERSION <= 1101) && !defined(BOOST_NO_CXX11_THREAD_LOCAL)
+#if ((_LIBCPP_VERSION <= 1101) || (_LIBCPP_VERSION == 7000)) && !defined(BOOST_NO_CXX11_THREAD_LOCAL)
 // This is a bit of a sledgehammer, because really it's just libc++abi that has no
 // support for thread_local, leading to linker errors such as
 // "undefined reference to `__cxa_thread_atexit'".  It is fixed in the

test/boost_no_unicode_literals.ipp

@@ -17,7 +17,7 @@ void quiet_warning(const CharT*){}
 
 int test()
 {
-#ifdef _<EFBFBD>_<EFBFBD>cpp_<EFBFBD>char8_<EFBFBD>t
+#ifdef __cpp_char8_type
    // The change to char8_t in C++20 is a breaking change to the std:
    const char8_t* c8 = u8"";
 #else