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configure.ac:35: warning: The macro `AC_PROG_LIBTOOL' is obsolete.
configure.ac:35: You should run autoupdate.
aclocal.m4:3465: AC_PROG_LIBTOOL is expanded from...
configure.ac:35: the top level
libtoolize: Consider adding 'AC_CONFIG_MACRO_DIRS([m4])' to configure.ac,
libtoolize: and rerunning libtoolize and aclocal.