Fix linking with -Wl,--no-undefined on Linux

libXvMCW uses dlopen and friends, therefore it must be linked against
libdl on systems where it exists as a separate library.

Signed-off-by: Yaakov Selkowitz <yselkowitz@users.sourceforge.net>
Reviewed-by: Alan Coopersmith <alan.coopersmith@oracle.com>
Signed-off-by: Alan Coopersmith <alan.coopersmith@oracle.com>
(cherry picked from commit 4dd267a9)