Better message that ffplay is not going to be built by printing out what
will be built.

Based on a patch by Moritz Barsnick.
Signed-off-by: rogerdpack <rogerpack2005@gmail.com>
Reviewed-by: Ganesh Ajjanagadde <gajjanag@mit.edu>
Signed-off-by: Timothy Gu <timothygu99@gmail.com>