Build ffmpeg-features on Conda binary dist (#2120)

Summary:
This commit enable ffmpeg-feature build on conda-based binary distribution on Linux and macOS.

It adds `ffmpeg` as build-time dependencies and enable the build with `BUILD_FFMPEG=1`.

Windows binaries, wheel-based binaries on Linux/macOS are not changed.

Pull Request resolved: https://github.com/pytorch/audio/pull/2120

Reviewed By: nateanl

Differential Revision: D33397473

Pulled By: mthrok

fbshipit-source-id: 67a23a40c0614c56fee60cc06a45f3265037f6df

packaging/torchaudio/build.sh

@@ -14,5 +14,5 @@ if [ "${USE_CUDA}" == "1" ] ; then
     fi
 fi
 shopt -u nocasematch
-
+export BUILD_FFMPEG=1
 python setup.py install --single-version-externally-managed --record=record.txt

packaging/torchaudio/meta.yaml

@@ -21,6 +21,7 @@ requirements:
     {{ environ.get('CONDA_PYTORCH_BUILD_CONSTRAINT', 'pytorch') }}
     {{ environ.get('CONDA_EXTRA_BUILD_CONSTRAINT', '') }}
     {{ environ.get('CONDA_CUDATOOLKIT_CONSTRAINT', '') }}
+    - ffmpeg >=4.1  # [not win]
 
   run:
     - python