How to specify dependencies with variants

[Jean-Michel Pouré]

Hello,

I am writing a port for MLT. MLT needs FFmpeg with all variants. How to
write it in the port. I wrote this, but obviously it is not working:

depends_lib         port:atk \
                    port:gavl \
                    port:opencv \
                    port:ffmpeg +a52 +avfilter +extvorbis +faac +faad
+gpl +lame +postproc +theora +x264 +xvid \
                    port:kdelibs4 \
                    port:kdelibs4-devel \
                    port:libquicktime \
                    port:libsamplerate \
                    port:pkgconfig \
                    port:sox \

Kind regards,
Jean-Michel