configure: Use $LIB<x> for linking a program, not $LTLIB<x>.

configure: Use $LIB<x> for linking a program, not $LTLIB<x>.

The gettext manual says:

     `@LIBINTL@' is for use without `libtool', `@LTLIBINTL@' is for
     use with `libtool'

It has similar wording for libiconv.

Since there's lots of libtool-related stuff in the PSPP makefiles, I
assumed that PSPP needed $LTLIB<x>.  But it now appears that this is
only correct for linking a libtool library, not for linking a
program that links against libtool libraries.

On GNU/Linux, both $LIB<x> and $LTLIB<x> expand to the empty string,
so I couldn't tell from experimenting there.

This should fix link failures with Mingw, reported by Harry Thijssen
<pspp@sjpaes.nl>.