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* Switch to cmake for build
* Hide symbols

* Use cmake for third party

* Apply patch to libmad

* Update gitignore

* Update docker test image

In #728, linux unit test switches to libsox provided by apt.
For CPU jobs this is fine because all the job steps share the same Docker container,
but on CPU job, each job step runs a script in a new Docker container, so
libsox installed in a step is not available to the subsequent steps.

To fix this, this PR moves the installation of libsox and sox to Docker build.

This is a part of PRs to add new "sox_io" backend #726, and depends on #718.

This PR adds `info` function to "sox_io" backend, which allows users to fetch some metadata of an audio file. 
At this moment, the information retrieved are;

 - Number of samples in the audio file
 - Sampling rate
 - Number of channels

* Clean up extension build mechanism and extension location

* Add back the switch to depend on external sox

* Remove print

* Fix

* Turn on tests when building through conda-build

* skip one failling test on windows.

* Switch to unittest

* Fix conda env activation

* add linux only condition

* Fix librosa installation

* conda-forge channel as a fallback

* Revert "conda-forge channel as a fallback"

This reverts commit 8c15f6638e34792640a3de3e057d9c2995425180.

* Try the pytorch way

* Use conda-forge

* Disable conda-forge

* Install future at setup env

* Move linux unittest CI job tools to dedicated directory

* Restore Linux scripts and move the updated souces to a new subdir

* Add back future

* Remove comments
Co-authored-by: Vincent Quenneville-Belair <vincentqb@gmail.com>
Co-authored-by: moto <855818+mthrok@users.noreply.github.com>

* Add test image with Kaldi executable and third_parties

* Use Ubuntu as bese image since pytorch/many-linux is huge

* wip

* Add test

* add caching

* fixup! add caching

* fixup! fixup! add caching