configure: Use LT_INIT from libtool 2 instead of deprecated AC_PROG_LIBTOOL

AC_PROG_LIBTOOL was replaced by LT_INIT in libtool 2 in 2008,
so it's time to rely on it.

Clears autoconf warnings:
configure.ac:36: warning: The macro `AC_PROG_LIBTOOL' is obsolete.
configure.ac:36: You should run autoupdate.
m4/libtool.m4:100: AC_PROG_LIBTOOL is expanded from...
configure.ac:36: the top level

Signed-off-by: Alan Coopersmith <alan.coopersmith@oracle.com>