Help with M4 and autoconf

[Jordi Gutiérrez Hermoso]

I'm revision at 14598:0d37fda09415, and I think the amount of repeated
code starting in configure.ac:2269 is kinda silly. I want to fix this.
So I wrote something like this:

    dnl
    dnl
    dnl OCTAVE_WARN_ABOUT_PROBLEMS(LIST-OF-PROBLEMS)
    dnl
    AC_DEFUN([OCTAVE_WARN_ABOUT_PROBLEMS], [
      m4_foreach([problem_found], [m4_set_list([$1])], [
        if test -n "$problem_found"; then
          AC_MSG_WARN([$problem_found])
          warn_msg_printed=true
        fi])
    ])

So now it's just a matter of populating the list, using m4_set_add, if
I am reading the autoconf manual correctly, which can be done with
something like

    m4_set_add([list_of_problems], [warn_$1])

Now, a lot of those warn_foo variables (macros?) are set in
m4/acinclude.m4:448, in the OCTAVE_CHECK_LIBRARY function (macro?),
some are set in various parts of configure.ac, and some I have no idea
where they are set (e.g. warn_fftw). Is this a bug? Is warn_fftw just
never set?

I'm floundering here. I don't really understand how to use M4. Can
someone give me some hints?

The reason I started doing all this is that I noticed that ARPACK
doesn't produce a clear warning at the end of ./configure if it's
missing, so I'm trying to fix this the "right" way, by eliminating
duplicate code and automating warnings about missing libraries a bit
more.

TIA,
- Jordi G. H.