configure: Add LT_INIT macro to enable libtool configuration.

Now we can use --enable-shared, --disable-shared, --enable-static,
--disable-static, --with-pic, and --without-pic configure flags to
choose whether we want dynamic or static library linking, etc.

See the link for details:
https://www.gnu.org/software/libtool/manual/html_node/LT_005fINIT.html

configure.ac

@@ -13,7 +13,7 @@ dnl checks for programs
 AC_PROG_MAKE_SET
 AC_PROG_CC
 AC_PROG_INSTALL
-AC_PROG_RANLIB
+LT_INIT
 
 dnl checks for header files
 AC_HEADER_STDC