Avoid redundant linking against libtool convenience libraries.

Avoid redundant linking against libtool convenience libraries.

Both libpspp.la and libpspp-core.la linked directly against libgl.la, and
the UI linked against both libpspp.la and libpspp-core.la.  This works fine
on ELF systems, but with Mach-O on Mac OS X it silently causes duplicate
symbols at runtime.  In particular there are two different copies of
rpl_optarg from libgl.la.  Different code sees different copies of these,
which causes a segfault at runtime whenever anyone invokes pspp with an
option that takes an argument.

Reported by Jeremy Lavergne <jeremy@lavergne.gotdns.org>, with debugging
assistance by Jeremy and by John Darrington.