Convert a .X mesh to a modern format
Blender appears to have a .X importer.  It's a plugin that ships with Blender, but is disabled by default, so you have to enable it in User Preferences.  See: https://blender.stackexchange.com/a/8749/18161

Blender also has a glTF exporter.  For now that's a separate download, from here: glTF-Blender-Exporter