Getting custom variables in a group

[Eric Abrahamsen]

I'm having an oddly hard time finding out programatically all the custom
variables in a custom group (recursively), or which group a custom
variable belongs to. I would have thought the latter would be a symbol
property, but it isn't.

Obviously `customize-group' must be able to find out, but when I
followed the code, the trail went cold in the widget jungle. I simply
don't see how it gets the list of options to display.

Right now I'm doing this, which I guess works okay, but it seems a bit hacky:

(let (target-syms)
  (mapatoms
   (lambda (sym)
     (when (and (custom-variable-p sym)
                (string-match-p "^gnus-\\|^message-"
                                (symbol-name sym)))
       (push sym target-syms))))
  target-syms)