mplayer: Fix external libdvdread and libdvdnav support

This sets paths to dvdread-config and dvdnav-config,
and configuration options to enable external libdvdread
and libdvdnav support.

Signed-off-by: Valentine Barshak <gvaxon@gmail.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>