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* Default to BUILD_SOX=1 in non-Windows systems

Since the adaptation of CMake and restricting to the static linking of libsox,
the build process has become much robust with libsox integration enabled.

This commit makes it default behavior to build libsox integration in non-Windows systems.
The build process still checks BUILD_SOX env var so, setting `BUILD_SOX=0` disables it.

* Switch to cmake for build
* Hide symbols

With this change, `BUILD_TRANSDUCER=1 python setup.py build_ext` now sees `-D_GLIBCXX_USE_CXX11_ABI=` in the compilation command. (Note: sox is C-only so it is not relevant to sox build process)

See also:
 - https://github.com/pytorch/text/pull/931
 - https://stackoverflow.com/a/55406930

* Use cmake for third party

* Apply patch to libmad

* Update gitignore

* Update docker test image

* Use env var for switching SoX build and default to not

* Update README

* Fix packaging/CI script

* Clean up extension build mechanism and extension location

* Add back the switch to depend on external sox

* Remove print

* Fix