Tweaks to compile cc-mode on XEmacs 21.4

Hi Alan,

I've successfully compiled the latest cc-mode on XEmacs 21.4.24 which
is also used to build packages, but it required a few minor tweaks.  I
can prepare a patch for the XEmacs cc-mode package (although they will
also work on GNU too) if you are interested but thought I'd describe
the issues first

1) Remove :safe as part of the defcustom as it's not recognised on
    21.4.24.

2) Explicitly specify the lower bound of {m,n} for (c-lang-defconst
c-symbol-key).

3) A documentation tweak for building on older versions of makeinfo
(e.g. CentOS 7.9)

    Add "GNU Free Documentation License" to the main menu in 
cc-mode.texi:250 to resolve
cc-mode.texi:7673: warning: node next `Mailing Lists and Bug Reports' in 
menu `Command and Function Index' and in sectioning `GNU Free 
Documentation License' differ
cc-mode.texi:7715: warning: unreferenced node `GNU Free Documentation 
License'
cc-mode.texi:7715: warning: node `Command and Function Index' is next 
for `GNU Free Documentation License' in sectioning but not in menu
cc-mode.texi:7715: warning: node `Mailing Lists and Bug Reports' is prev 
for `GNU Free Documentation License' in sectioning but not in menu
cc-mode.texi:7715: warning: node `Top' is up for `GNU Free Documentation 
License' in sectioning but not in menu
cc-mode.texi:250: node `Top' lacks menu item for `GNU Free Documentation 
License' despite being its Up target
cc-mode.texi:7722: warning: node prev `Command and Function Index' in 
menu `Mailing Lists and Bug Reports' and in sectioning `GNU Free 
Documentation License' differ
make: *** [cc-mode.info] Error 1

I've been using these changes on 21.4.24 and 21.5.35 so far without
any issues.  Any thoughts?

Best,
Richard

p.s. Thanks for all the great work on cc-mode - for me the defaults
Just Work whether I'm using GNU or XEmacs.