Require 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.

configure.ac:35: warning: The macro `AC_LIBTOOL_WIN32_DLL' is obsolete.
configure.ac:35: You should run autoupdate.
aclocal.m4:11889: AC_LIBTOOL_WIN32_DLL is expanded from...
configure.ac:35: the top level
configure.ac:35: warning: AC_LIBTOOL_WIN32_DLL: Remove this warning and the call to _LT_SET_OPTION when you
configure.ac:35: put the 'win32-dll' option into LT_INIT's first parameter.
./lib/autoconf/general.m4:2434: AC_DIAGNOSE is expanded from...
aclocal.m4:11889: AC_LIBTOOL_WIN32_DLL is expanded from...
configure.ac:35: the top level
configure.ac:36: warning: The macro `AC_PROG_LIBTOOL' is obsolete.
configure.ac:36: You should run autoupdate.
aclocal.m4:3465: AC_PROG_LIBTOOL is expanded from...
configure.ac:36: the top level