Given that currently `filter_audio` doesn't help qTox users to improve
quality of their conversations, there's little to no point in making
qTox build with it by default. This might change when (if?)
`filter_audio` support in qTox gets improved/fixed.
BREAKING CHANGE: Disabling of `filter_audio` was done by passing
`DISABLE_FILTER_AUDIO=YES` to `qmake`. With this change `filter_audio`
is disabled by default, and in order to enable it,
`DISABLE_FILTER_AUDIO=NO` has to be passed to `qmake`.