Sorting the jack build-dependency mess

[Felipe Sateler]

I'm not happy with the way currently we need to add alternative
build-depends on the different libjack implementations. Possible ways
out of this problem:

1. Make all jack implementations provide: libjack-dev. This leaves us
with unversionable build-depends.
2. Create a new virtual package, like we did with the binary. This
means many sourceful uploads.
3. Create a dummy package libjack-dev that depends on the default
implementation, but allows libjack2 or any other future
implementation. Could eventually allow versioned depends, although
with a weird version number.

I'm not quite sure losing the versioning is bad... supposedly jack
hosts cannot rely on anything newer than 0.116.0 being present at
runtime. So I'd incline towards either 1 or 3, which are the less work
involved.

What do you think?

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Saludos,
Felipe Sateler