When running autoreconf, the script scans configure.ac for
AC_PROG_LIBTOOL to determine if libtool is being used. libtool.m4
provides AM_PROG_LIBTOOL as a compatibility alias for AC_PROG_LIBTOOL,
but autoreconf doesn't check for this, so autoreconf may not run
libtoolize in some cases when it's actually needed.
AM_PROG_LIBTOOL is deprecated according to Libtool's documentation,
and is not mentioned at all in Autoconf's or Automake's documentation
(AC_PROG_LIBTOOL is). I believe the proper solution is to switch the
Autoconf input file to use AC_PROG_LIBTOOL instead of AM_PROG_LIBTOOL.