Replace AC_PROG_LIBTOOL with LT_INIT in configure.ac

AC_PROG_LIBTOOL is a deprecated name for older versions of LT_INIT.[1]

1. http://www.gnu.org/software/libtool/manual/html_node/LT_005fINIT.html
Merge pull request #116 from mortehu/master

Replace AC_PROG_LIBTOOL with LT_INIT in configure.ac