hopefully fix FS#695:

use AC_PROG_LIBTOOL and AC_DISABLE_SHARED instead of
LT_INIT([disable-shared]).  LT_INIT may be the new way of doing this, but the
other are not yet deprecated and I could not get LT_INIT to work

tested with libtool 2.2.6 and 1.5.26

[[Imported from SVN: r5779]]