Make it possible to build as a meson subproject and make cmoka an optionnal dependency

[thiblahute]

This adds a meson option to allow disabling tests.

[andrew-elder]

I know nothing about meson, although changes look to be ok to me. Can some else please provide some additional feedback?

[thiblahute]

You can find additional information about subprojects here btw.

This allows us to build libavtp as part of the GStreamer build if it is not available on the system.

[thiblahute]

Also not that since meson 0.47.0 there is a new option type called features which would allow us to make cmoka optional dependency handling nicer but I didn't want to bump the dep (and 0.47 requires python3>=3.5 which is not present on the CI system you use).

[aguedes]

Hi @thiblahute, thanks for the PR. I'll review it by EOW.

[aguedes]

Hi @thiblahute, thanks for this updated version. I'm merging it now. Regarding the 'features' option type introduced in meson 0.47.0, yes, it would make the cmocka dependency handling nicer indeed. If you're up to it, feel free to update our meson and python versions in CI and refactor the dependency handling. I'd be glad to merge those patches! :)