configure.ac: use LT_INIT if available
AC_PROG_LIBTOOL is deprecated. quiets a warning from autoconf 2.71:
configure.ac:12: warning: The macro `AC_PROG_LIBTOOL' is obsolete.
@@ -9,7 +9,8 @@ dnl === automake >= 1.12 requires this for 'unusual archivers' support.
dnl === it must occur before LT_INIT (AC_PROG_LIBTOOL).
m4_ifdef([AM_PROG_AR], [AM_PROG_AR])
-AC_PROG_LIBTOOL
+dnl === AC_PROG_LIBTOOL is deprecated.
+m4_ifdef([LT_INIT], [LT_INIT], [AC_PROG_LIBTOOL])
AC_PROG_SED
AM_PROG_CC_C_O
