Use weak loading of libjack

[tresf]

WeakJack is a third party tool that abstracts the JACK Application Binary Interface for use with various jack versions. It helps make LMMS a portable application on Linux.

• This used to be part of Add AppImage support to Linux builds #3688. It was separated to make tracking (blaming, cherry-picking, submoduling) easier.
• This is 100% 3rd-party code and should eventually be submoduled.

[tresf]

Travis-CI passes for Linux and Windows. I'm not waiting for Apple, this has been tested several times in the other PR. Merging.

[lukas-w]

Can we move the third-party code to a dedicated directory, such as src/3rdparty, similar to what I've done in #3873 or #3475? This will make it clear to everyone that it's not part of LMMS, and we can easily exclude it from tools like automatic code formatting.

Also, maybe we should add a file or a comment somewhere that documents the upstream source of the code.

[tresf]

@lukas-w I agree fundamentally, although I see little benefit in doing refactoring in stable-1.2 branch. We could triage these refactoring items into a new milestone so that we don't forget them? ArashPartow/exprtk is another candidate.

[lukas-w]

Sure 👍