[sugar] jhbuild changes

[Marco Pesenti Gritti]

Hello,

I landed a few sugar-jhbuild changes last weekend. Highlights:

* You are better off doing a clean checkout/build.
* We are now pulling jhbuild upstream directly instead of using an
internal copy. Misc cleanups of the scripts and of the directory
structure.
* I reworked the sanity check do be distribution specific. It should
be much easier to install the missing dependencies now because you
will get an actual list of packages to install (Ubuntu and Fedora are
supported so far). Also we can reduce the number of packages built by
specifying to use system packages for some dependencies in the sysdeps
definition.
* For distributions which are not supported yet, stuff will work as
usual, except you won't get any dependency check.
* If you want to support for a new distribution you have to add a
little logic to scripts/sysdeps.py to recognize it and a new sysdeps
definition in config/sysdeps. Should be very easy.

Marco