[PATCH] Make build of jack support optional

Hi,
during build, jack support is currently automagically detected. This is fine for most users, but suboptimal for source-based Linux distributions (Gentoo) which sometimes need to disable jack support even when jack is present in system.

So I created following patch (applies to today's trunk), which introduces new cmake cached variable BUILD_JACK, which can be used to control building of jack support. It still defaults to automatic detection.