Fix __clang__/__clang_major__ confusion when trying to define language feature

macros in terms of Clang major version number.

Fixes #168.

test/replace.cpp

@@ -37,7 +37,7 @@ namespace deduction {
 // MSVC and older Clangs produce hard errors here, so ill_formed does not
 // work.
 #if defined(__cpp_char8_t) && !defined(_MSC_VER) &&                            \
-    (!defined(__clang__) || 16 <= __clang__)
+    (!defined(__clang_major__) || 16 <= __clang_major__)
 char const empty_str[] = "";
 
 template<typename T>