The 2.13 macro is actually AC_MSG_WARN but when I tried using that it still failed while it was contained in the m4_if. Moving the line out of the m4_if, having the $0 in there seems to cause some kind of infinite recursion (process dies eventually from lack of memory).
Well, m4_if isn't even supported on autoconf-2.13 and AC_MSG_WARN is for configure-time warnings, not autoconf-time warnings - which is what we wanted this to be.
Regardless, please try HEAD.
I will also note that libtool-1.5 and higher *require* autoconf-2.50 or higher. So, I would expect that we should drop autoconf-2.13 support sometime in the near future via AC_PREREQ(2.50).
(In fact for releases, autoconf-2.13 is certainly way too buggy.) -- justin
