Depending on port flag?

[Henry Hu]

Recently, I tried to use mpv with VAAPI. The default mpv port does not
enable VAAPI, so I enabled and installed it from ports.
However, mpv still can't use VAAPI. After some investigation, it turns out
that it depends on ffmpeg with VAAPI enabled, and the default ffmpeg also
has it disabled. After rebuilding ffmpeg with VAAPI, it works.

So if mpv's VAAPI flag is enabled, it should depend on ffmpeg's VAAPI flag.
Is there a formal method to depend on another port's flag currently? There
are hackish method like "depend on this file which is only installed when
that flag is enabled", but there should be a better method.