ac_cv_sys_largefile_opts undocumented?

Hello,
I am not 100% sure whether this should go to a gnulib list or to an autoconf list,
but anyway, ac_cv_sys_largefile_opts (from a subroutine in
AC_SYS_LARGEFILE) is not documented in either gnulib manual, or in
autoconf manual. (It is documented in the source).

Thus I got questions whether a patch for a build system I submitted for 
a project is OK, as it uses an undocumented variable (thus, perhaps,
unstable).

By the way, ac_cv_sys_largefile_opts isn't fun to use - as sometimes
it's a plain text, and sometimes flags which should be added to CFLAGS.
So one has to write things like

AS_CASE([$ac_cv_sys_largefile_opts],
  ["none needed"], [],
  ["support not detected"], [],
  dnl otherwise it is a compiler flag
  [LARGEFILECFLAGS="$ac_cv_sys_largefile_opts"]
  )

which are unnecessarily complicated.

Please advice, and possibly fix.

Best,
Dima

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