Build with ffmpeg only on Linux - #4463

Hmm.
wouldn't we want to be able to build it on a macOS?
If I understand correctly, the linking segfault is gone when pytorch is compiled from source; maybe we can just add that to the documentation?

Or even better, maybe add a flag that would be off by default (on a mac or not) so that we can do
WITH_FFMPEG=1 python setup.py isntall

Hmm.
wouldn't we want to be able to build it on a macOS?
If I understand correctly, the linking segfault is gone when pytorch is compiled from source; maybe we can just add that to the documentation?

Or even better, maybe add a flag that would be off by default (on a mac or not) so that we can do
WITH_FFMPEG=1 python setup.py isntall

This PR is against the release branch.