Adding to TeX-clean-default-intermediate-suffixes

[Greg Bognar]

In my config, I have

(setq TeX-clean-default-intermediate-suffixes
        '("\\.aux" "\\.bbl" "\\.bcf" "\\.blg" "\\.brf" "\\.dvi" "\\.ent"
          "\\.fdb_latexmk" "\\.fls" "\\.fmt" "\\.fot" "\\.glo" "\\.gls"
          "\\.synctex\\.gz" "\\.idx" "\\.ilg" "\\.ind" "\\.lof" "\\.log"
          "\\.lot" "\\.nav" "\\.out" "\\.rel" "\\.rip" "\\.snm" "\\.tags"
          "\\.toc" "\\.url" "\\.vrb" "\\.run\\.xml"))

I added a number of extra extensions to the default value for files created by
RefTeX, Beamer, Synctex, etc.  I want to clean up all these files once I'm done
with a document.

However, this does not work.  E.g., it leaves RefTeX's .rel files around.  Why
is that?  Is it because TeX-clean-default-intermediate-suffixes is defined by a
defvar in tex.el, rather than a defcustom?  Shouldn't this be a customizable
option in the first place?  If not, what is the proper way to add the extra
extensions?

Thanks!