New matrix for 5 jobs in total.
Updates our checks to bionic & xenial. Trusty could also be
included but it is a bit outdated at this point.
I use bionic to pull a more recent clang-format and use a dedicated job
just to check formatting. An alternative would be to conditionally run
the formatting script only on bionic.
I don't think that there is a better way to expand a `dist` x `compiler`
matrix (except using docker).
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
