In C++20 the u8 literal produces a char8_t[] not a char[].

Fixes https://github.com/boostorg/config/issues/276.

test/boost_no_unicode_literals.ipp

@@ -17,7 +17,12 @@ void quiet_warning(const CharT*){}
 
 int test()
 {
+#ifdef _<>_<EFBFBD>cpp_<70>char8_<38>t
+   // The change to char8_t in C++20 is a breaking change to the std:
+   const char8_t* c8 = u8"";
+#else
   const char* c8 = u8"";
+#endif
   const char16_t* c16 = u"";
   const char32_t* c32 = U"";
   quiet_warning(c8);