Avoid using the OSS emulation layer to fetch the master volume,
and instead share the audioio code with OpenBSD.
Avoid a failed read on the master control's mute enum if it can't
be found, because sometimes there isn't one.
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
This matches the conditional compilation in the code and is more correct than
distinguishing linux vs. non-linux (which breaks on Debian’s kFreeBSD and hurd
variants).
Thanks to sdk for providing an OpenBSD 6.5 environment for verification. This
has not been tested on DragonFlyBSD.
related to #352
