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configure.ac:18: warning: The macro `AC_PROG_LIBTOOL' is obsolete.
configure.ac:18: You should run autoupdate.
aclocal.m4:3640: AC_PROG_LIBTOOL is expanded from...
configure.ac:18: the top level

libtoolize: Consider adding 'AC_CONFIG_MACRO_DIRS([m4])' to configure.ac,
libtoolize: and rerunning libtoolize and aclocal.

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