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configure.ac:39: warning: The macro `AC_PROG_LIBTOOL' is obsolete. configure.ac:39: You should run autoupdate. aclocal.m4:3640: AC_PROG_LIBTOOL is expanded from... configure.ac:39: the top level libtoolize: Consider adding 'AC_CONFIG_MACRO_DIRS([m4])' to configure.ac, libtoolize: and rerunning libtoolize and aclocal.