There are quite a few references to an autoconf macro to "unprecious" a variable. However, the simplest substitution will not work with libtool, causing it to get stuck during autoreconf. Fixing this requires only attempting the substitution if _AC_PRECIOUS_VARS is actually set:

AC_DEFUN([AX_UNPRECIOUS], [
        m4_ifdef([_AC_PRECIOUS_VARS], [m4_define([_AC_PRECIOUS_VARS], m4_bpatsubst(_AC_PRECIOUS_VARS, [$1], []))])
])