Import a few entries from RetroArch.
I also have a few entries RetroArch lacks; they are the following:
RETROK_BACKQUOTE / XK_dead_acute
RETROK_CARET / XK_dead_circumflex
RETROK_RALT / XK_ISO_Level3_Shift
RETROK_KP_PERIOD / XK_KP_Separator
RetroArch doesn't seem to like having two entries for the same RETROK_xxx, so I'm not going to push for any change.