configure: Use LT_INIT from libtool 2 instead of deprecated AC_PROG_LIBTOOL

Merged Alan Coopersmith requested to merge alanc/libxt:config into master

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

Clears autoconf warnings: warning: The macro `AC_PROG_LIBTOOL' is obsolete.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

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