configure: cocoa: link to libarclite

libarclite provides method stubs for the Subscripting headers added in
0407869ae3. This allows to correclty build mpv on OSX 10.7 (I had tested that
commit with OSX 10.8 running 10.7 SDK).

It seems on 10.8 this option does't make any difference in the linked libraries
(checked with otool -L) so I just add it unconditionally.

Warning: This doesn't mean mpv moved to ARC. To do that one would have to add
`-fobjc-arc` to the cflags.