Automatically link every program against libintl and libiconv.

Automatically link every program against libintl and libiconv.

On glibc systems, libiconv and libintl are both built into libc.  That
means that it's very easy to forget to link programs against them,
because it still builds OK on GNU/Linux.  However, on other systems
they are usually separate libraries and forgetting to link against
them breaks the build.

This commit switches from previous PSPP practice, in which these
libraries were linked explicitly to every binary that requires them,
to a simplified practice where they are in LIBS so that every binary
is automatically linked to them.  This has no disadvantage on
GNU/Linux, our primary target, and the advantage that on other targets
PSPP works more consistently.