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`.
Beside making it work again, there are some improvements:
Linux side:
* use minimal supported Qt version, currently it's 5.3
* use minimal supported GCC version, currently 4.8
* use newer ffmpeg version to compile against - 2.8.5
* compile qTox 2 times, first run without support for optional dependencies
* use ubuntu 14.04 for building (less deps needs to be compiled/added from PPAs)
OSX:
* add osx to build targets
