make pulseaudio an optional dependency, follow best practices

For my thoughts about optional dependencies, see
https://michael.stapelberg.ch/posts/2019-05-23-optional-dependencies/

This commit follows the best practices outlined in that article:

1. The travis config was modified to verify both code paths build and link/don’t
   link against pulseaudio.

2. If pulseaudio is missing, the build fails until packagers explicitly pass a
   --disable flag. In practice, I think the only situation when this flag should
   be set is in source-based linux distributions where users can express
   package-level compilation preferences (e.g. Gentoo USE flags).

3. The --version output now reflects the status of the optional dependency.

fixes #359

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