Quiet gcc and clang warnings

gcc ignores the c++17 standard advisory in some versions and simply
doesn't warn about unused private variables.  Clang and MSVC do warn
about unused private variables and do follow the standard

include/locale_io.h

@@ -50,7 +50,7 @@ private:
     // The locale in use before switching to the "C" locale
     // (the locale can be set by user, and is not always the system locale)
     std::string m_user_locale;
-    #ifdef __GNUC__
+    #ifndef __clang__
     // [[maybe_unused]] attribute is ignored by Gcc but generates a warning.
     wxLocale*   m_wxLocale;
     #else