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
