Give up on a cross-platform solution to non-ascii filenames.

Sometime it might be worth giving another go at it, but for now I
just put conditional compilation in.

Fixes https://gitlab.com/kicad/code/kicad/issues/5024

common/settings/json_settings.cpp

@@ -159,7 +159,12 @@ bool JSON_SETTINGS::LoadFromFile( const std::string& aDirectory )
     else
     {
         wxString dir( aDirectory.c_str(), wxConvUTF8 );
+#ifdef __WINDOWS__
         path.Assign( dir, m_filename, getFileExt() );
+#else
+        wxString name( m_filename.c_str(), wxConvUTF8 );
+        path.Assign( dir, name, getFileExt() );
+#endif
     }
 
     if( !path.Exists() )